Key Takeaways
- Most lumps on dogs are harmless, such as fatty lipomas, cysts, warts, and skin tags. But 20% to 40% of skin lumps that vets send to a lab turn out to be cancer.[1]
- You can't tell by looking or feeling. Even cancer specialists can't. Mast cell tumors, the most common skin cancer in dogs, can feel exactly like a soft fatty lump.[1][2]
- Use the "pea for a month" rule. Any lump the size of a pea (1 cm) or larger that has been there for a month should be tested.[1]
- The test is quick. A fine needle aspirate is fast, low cost, and far less invasive than a biopsy, and it often gives an answer.[1]
- Small is curable. Small cancers caught early are often cured with surgery alone. Waiting makes surgery bigger and less likely to work.[1]
Have a vet check the lump within a few days if it:
- Is pea-sized (1 cm) or larger and has been there a month.[1]
- Is growing, especially quickly.[2]
- Changes size from day to day, or gets red and puffy when touched. This is typical of a mast cell tumor.[3]
- Is open, bleeding, or oozing.
- Feels firm and attached to the tissue underneath instead of moving freely.
- Is in the mouth, on a toe or nail bed, or along the belly near a nipple of a female dog.[2]
- Is dark or black and new or changing.[2]
- Is painful, hot, or bothering your dog.
Go to an emergency vet now if lumps or swelling appear suddenly on the face or throat, especially with vomiting, weakness, or trouble breathing. This can be a severe allergic reaction.
Types of Lumps and Bumps on Dogs
| Lump | What it usually looks and feels like | Typical dog | Cancer? |
|---|---|---|---|
| Lipoma (fatty tumor) | Soft, squishy, moves freely under the skin. Mostly on the chest, belly, and upper legs.[2] | Older, heavier dogs[4] | No |
| Histiocytoma | A single raised, pink or red, hairless "button" that appears quickly. May ulcerate.[2] | Young dogs, usually under 3½ | No. Usually goes away in 2 to 3 months. |
| Sebaceous or follicular cyst | A round, firm bump in the skin that may release gray, white, or yellow cheesy material.[2] | Any dog | No |
| Sebaceous adenoma | Small, pale, wart-like or cauliflower-like bumps, often several, commonly on the head.[2] | Older dogs, especially Cocker Spaniels | No |
| Wart (papilloma) | Rough, cauliflower-like bumps. Often several around the mouth in young dogs, single in older dogs.[2] | Young dogs and seniors | No. Usually clears on its own. |
| Skin tag | A soft flap of skin on a stalk.[2] | Older dogs, often large breeds | No |
| Abscess | A warm, painful swelling that comes up over a day or two, often after a bite or wound. May burst and drain pus. | Any dog | No, but needs treatment |
| Hives or insect bite | Raised welts or a single swollen bump that appears within minutes to hours. | Any dog | No |
| Mast cell tumor | Can look like anything: a soft lump, a red raised bump, or an ulcer. May swell and shrink.[3] | Average age 8 to 9. Boxers, Boston Terriers, Pugs, Bulldogs, Golden Retrievers.[3] | Yes |
| Soft tissue sarcoma | A firm lump under the skin that slowly gets bigger and feels attached. Usually not painful.[2] | Middle-aged and older dogs | Yes |
| Melanoma | A dark brown or black lump. Skin ones are often benign. Those in the mouth or on a toe are usually malignant.[2] | Older dogs | Sometimes |
| Squamous cell carcinoma | A crusted, raised, or ulcerated sore that doesn't heal, often on sun-exposed pale skin or a toe.[2] | Older dogs. White, short-haired breeds. | Yes |
This table describes typical cases. Any of these can look like another, which is why testing matters.
Are Cancerous Lumps on Dogs Hard or Soft?
They can be either. It's true that lipomas are usually soft and movable, and that many cancers feel firm and fixed in place. But the exceptions are common enough that feel alone is not safe to rely on:
- Mast cell tumors often feel soft and can be mistaken for lipomas.[2]
- Soft tissue sarcomas can sit under the skin for months looking like a harmless lump. In one published example, a dog with more than 10 confirmed lipomas had a new lump that turned out to be a sarcoma.[1]
- Cancerous lumps usually don't hurt, so a dog acting normal tells you nothing.
This is why vets use a needle test instead of a "wait and see" approach.[1]
Sudden Lumps Under a Dog's Skin
A lump that appears overnight or within a few days is more often inflammation than cancer. Common causes:
- Hives or an allergic reaction: many raised welts, often with a swollen face. See hives in dogs.
- Insect sting or tick bite: a single swollen, sometimes itchy bump. See ticks on dogs.
- Abscess: a hot, painful swelling, often after a bite or puncture. See abscesses in dogs.
- Vaccine or injection reaction: a small firm lump at the injection site that fades over a few weeks.
- Bruise or fluid pocket (hematoma or seroma) after a knock or surgery.
- Histiocytoma in a young dog.[2]
Cancer can also seem to appear suddenly. Mast cell tumors can grow quickly and can swell within hours when bumped or scratched.[3] If a sudden lump is still there after a week or two, have it tested.
The Most Common Lumps, Explained
Lipomas (fatty tumors)
Lipomas are benign tumors made of fat. They're soft, usually move freely, and are most common on the chest, belly, and upper legs of older, overweight dogs.[2] In a study of about 384,000 UK dogs, roughly 1 in 50 was diagnosed with a lipoma in a single year, with Weimaraners, Dobermans, German Pointers, Springer Spaniels, and Labradors at highest risk.[4] Most never need treatment. Surgery is used when a lipoma is large, growing, or in the way, for example in an armpit. Rarely, a fatty tumor grows into nearby muscle (infiltrative lipoma) or is malignant (liposarcoma).[2] There's no proven way to shrink a lipoma with diet, supplements, or creams.
Histiocytomas
These pink, hairless "button" lumps show up quickly on young dogs, most often under 3½ years old. Bulldogs, Boxers, Boston Terriers, Scottish Terriers, and Greyhounds get them more often. They're benign and usually disappear on their own within 2 to 3 months.[2] A vet should still confirm it with a needle sample, because a mast cell tumor can look the same.[3]
Sebaceous cysts and adenomas
Cysts are sacs in the skin filled with thick gray, white, or yellowish material. Don't squeeze or pop them. That can push the contents into the surrounding tissue and cause severe inflammation.[2] Sebaceous adenomas are small, wart-like growths of the oil glands, common in older dogs. Both are benign and are only removed if they get infected, bleed, or bother the dog.[2] See dog skin cysts and dog skin pimples.
Warts and skin tags
Warts are caused by a virus that spreads between dogs by contact and shared items like bowls and bedding. They usually go away on their own.[2] Skin tags are soft, stalk-like growths in older dogs. Removal is optional for both.[2] See dog skin warts.
Mast cell tumors
Mast cell tumors make up 7% to 21% of skin tumors in dogs.[3] They're hard to spot because they can look like a lipoma, a wart, an insect bite, or an allergic bump.[2][3] They release histamine, so they may swell, redden, or itch, especially after being handled. Avoid squeezing or rubbing a suspicious lump.[3] Most low-grade tumors are cured with surgery. High-grade tumors are aggressive and usually need chemotherapy as well.[3]
Other skin cancers
Soft tissue sarcomas are firm lumps under the skin that invade the tissue around them. They rarely spread through the body but often come back unless removed with a wide margin.[2] Melanomas in the mouth and on the toes, and squamous cell carcinomas on sun-exposed skin and toes, also need early surgery.[2] See cancer in dogs.
How to Check Your Dog for Lumps and Track Them
- Do a monthly "lump check." Run your hands over your dog's whole body, including the armpits, groin, between the toes, under the tail, and inside the mouth.
- Measure any lump with a ruler or compare it to a pea, a grape, or a coin. Write down the size and the date.[1]
- Take a photo with a coin next to the lump for scale.
- Mark the spot on a simple body sketch, so you and your vet can find it again.
- Recheck every 2 to 4 weeks. Book a vet visit if it reaches pea size and has been there a month, or if it changes.[1]
What the Vet Will Do
- Fine needle aspirate (FNA): A small needle collects cells from the lump to look at under a microscope. It's quick, low cost, and much less invasive than a biopsy. It identifies many lumps, including lipomas, and it's the usual first test for mast cell tumors.[1][3]
- Biopsy: If the needle sample doesn't give a clear answer, the vet removes a small piece of the lump for a lab to examine. A biopsy also shows how aggressive a tumor is (its grade).[1][3]
- Staging tests for confirmed cancers, such as checking nearby lymph nodes, chest X-rays, or ultrasound.
Ask for the lump to be tested before it's removed. Knowing what it is lets the surgeon plan the right size of surgery the first time. Removing an unknown lump with too small a margin often leads to a second surgery.[1]
Treatment
- Benign lumps: Most are simply monitored. They're removed if they grow, get infected, bleed, or get in the way.[2]
- Abscesses and infections: Draining, cleaning, and antibiotics from your vet.
- Cancerous lumps: Surgery is the main treatment. Malignant tumors often need 2 to 3 cm of healthy tissue removed all around them.[1] Radiation, chemotherapy, or newer targeted drugs may be added, depending on the tumor type and grade.[3]
Supplements, creams, and home remedies don't remove tumors, and delaying a diagnosis to try them can let a treatable cancer grow.
What the Research Shows
- A meaningful share of lumps are cancer. Skin lumps make up 25% to 43% of the samples vets send to labs, and 20% to 40% of those are malignant.[1]
- Lipomas are strongly tied to age and weight. Dogs aged 9 to 12 had about 17 times the odds of a lipoma compared with dogs aged 3 to 6, and dogs at or above their breed's average weight had about twice the odds.[4]
- Size matters with mast cell tumors. Tumors larger than about 3 cm (1¼ inches) are linked to shorter survival.[2] Average survival is more than 2 years for low-grade tumors and less than 4 months for high-grade ones.[3]
- Some sarcomas return. Only about 10% of fibrosarcomas spread, but more than 70% come back within a year of surgery, which is why wide removal the first time matters.[2]
Want a vet's opinion on a lump today?
You can ask a licensed veterinarian online now and send a photo. An online vet can help you decide how soon to go in, but only a needle test or biopsy can tell you what a lump is.
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Related: dog skin conditions · sores and scabs on dogs · lumps on a dog's neck
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Frequently Asked Questions
Should I be concerned if I find a lump on my dog?
Most lumps are harmless, but you can't tell which ones by looking. Measure it, write down the date, and have your vet test any lump that is pea-sized or larger and has been there a month, or any lump that is growing, changing, or bleeding.[1]
Are cancerous lumps on dogs hard or soft?
They can be either. Many cancers feel firm and fixed in place, but mast cell tumors, the most common skin cancer in dogs, often feel soft like a fatty lump. Only a needle sample or biopsy can tell.[1][2]
What does a cancerous lump look like on a dog?
There's no single look. Warning signs include fast growth, a lump that changes size, redness, an open or bleeding surface, a dark color, or a firm lump that feels attached underneath. Some cancers look completely ordinary.[2][3]
How can I tell if my dog's lump is a cyst or a tumor?
Cysts are usually round, sit in the skin, and may release thick white, gray, or yellow material. Tumors are solid. In practice they can look the same, so vets confirm with a fine needle aspirate.[1][2]
What is a lipoma, and does it need to be removed?
A lipoma is a benign fatty tumor, usually soft and movable, common in older and overweight dogs. Most don't need removal. Surgery is recommended if it's large, growing quickly, or interfering with movement.[2][4]
Can I pop my dog's cyst?
No. Squeezing a cyst can push its contents into the surrounding tissue and cause painful inflammation or infection. If a cyst bursts on its own, keep it clean and see your vet.[2]
Do histiocytomas go away on their own?
Usually, yes. Most histiocytomas in young dogs shrink and disappear within 2 to 3 months. Have your vet confirm the diagnosis first, and go back if it's still growing or hasn't gone after 3 months.[2]
Why did a lump suddenly appear on my dog?
Sudden lumps are often hives, an insect sting, an abscess, or a reaction to an injection. Some tumors, including mast cell tumors and histiocytomas, can also appear quickly. If it's still there after a week or two, have it tested.[2][3]
What is a fine needle aspirate?
It's a quick test where the vet uses a small needle to collect cells from the lump and looks at them under a microscope. It's much less invasive than a biopsy. If the result isn't clear, a biopsy is the next step.[1]
How long can a dog live with a mast cell tumor?
It depends on the grade. Dogs with low-grade tumors that are fully removed often live more than 2 years and many are cured. High-grade tumors are aggressive, with average survival under 4 months.[3]
References
- Ettinger S. Veterinary Oncology: What to Do With Lumps and Bumps on Dogs and Cats. Today's Veterinary Practice. July/August 2017.
- Villalobos AE. Tumors of the Skin in Dogs. Merck Veterinary Manual (Pet Owner Version). Updated April 2026.
- Weir M, Williams K, Pinard C, Downing R. Mast Cell Tumors in Dogs. VCA Animal Hospitals. Updated February 2024.
- O'Neill DG, Corah CH, Church DB, Brodbelt DC, Rutherford L. Lipoma in dogs under primary veterinary care in the UK: prevalence and breed associations. Canine Genetics and Epidemiology. 2018;5:9.
This guide is for general information and is not a substitute for an exam by your veterinarian. Dog Health Guide is owned and operated by Web Ventures of New York, Inc.
