Lump or Mass in Cats — Symptom Guide

Found a lump on your cat? This guide helps you describe what you see, assess urgency, understand common causes (abscess, mammary tumor, injection-site sarcoma, lipoma) and know next steps.

Quick Assessment

• Is this an emergency? Yes if the lump is rapidly growing, painful, ulcerated/bleeding, associated with fever (>39.2°C / 102.5°F), trouble breathing, or your cat is very unwell. Otherwise: urgent veterinary attention is recommended within days to a week because lumps in cats are more often malignant than in dogs.
• Most common cause: cat fight abscesses and inflamed bite wounds are the most frequent palpable lumps in outdoor/indoor-outdoor cats. For indoor-only cats, benign fatty masses and mammary nodules can be common.
• When to see a vet: Within 48–72 hours if painful, warm, leaking, or your cat has fever or lethargy. Within 1–2 weeks for any firm lump that isn’t shrinking, or sooner if it grows, becomes fixed/ulcerated, or your cat shows systemic signs.

What this symptom looks like

A "lump" or "mass" is any localized swelling under the skin, within the skin, or attached to deeper tissues. Owners may describe it as:

• A soft, squishy bump (may be a cyst or fat)
• A warm, painful, fluctuant swelling that may drain pus (suggests abscess)
• A firm, immovable nodule under the skin (can be scar tissue, tumor, or injection-site sarcoma)
• A raised ulcerated sore or a lump that bleeds
• One or several lumps along the belly (common for mammary tumors)

Possible causes (ranked from most to least likely overall)

Note: In general, lumps in cats are more likely to be malignant than similar lumps in dogs. That means early evaluation and tissue sampling (fine needle aspiration or biopsy) is especially important in cats so appropriate treatment can begin without delay.

Sources: Merck Veterinary Manual; Cornell Feline Health Center.

Why lumps in cats are more often malignant than in dogs (brief explanation)

Population-level studies and clinical experience show a higher proportion of cutaneous and subcutaneous masses in cats are malignant compared to dogs. The reasons are multifactorial (differences in species-specific tumor biology and common tumor types such as mammary carcinoma and injection-site sarcoma in cats). Because of that higher risk, veterinarians commonly recommend earlier sampling (cytology or biopsy) in cats rather than watchful waiting.

Decision tree: quick practical guide

• If lump is soft, mobile, not painful + hasn’t changed in 3+ months → likely benign (lipoma or scar) → action: photograph, measure, recheck in 2–4 weeks, schedule vet visit if growth or change.

• If lump is fluctuant, warm, painful, +/- draining pus + history of a bite fight → likely abscess → action: see vet within 48 hours for draining, flushing, and antibiotics; don’t squeeze at home.

• If lump is firm, immobile, grew over weeks to months, or located at a previous injection site → possible injection-site sarcoma or other tumor → action: see a vet urgently (within days) for FNA/biopsy; early specialist referral may be advised.

• If lump is on or near mammary chain in intact or previously intact female + gritty/firm and possibly ulcerated → possible mammary tumor → action: see vet within days for evaluation and staging (bloodwork, imaging) and discussion of removal.

• If lump is accompanied by fever (>39.2°C / 102.5°F), weight loss, lethargy, trouble breathing or swallowing → systemic illness likely → action: emergency vet within 24 hours.

Home assessment steps (what to check and how)

Write down everything above to share with your veterinarian.

When it's an emergency — immediate veterinary care

Seek emergency care if any of the following are present:

• Rapid growth over days (whole lump becomes much larger)
• Lump is ulcerated, bleeding, or has a bad-smelling discharge
• Signs of systemic illness: fever >39.2°C (102.5°F), severe lethargy, collapse
• Pain so severe the cat won’t move or eat
• Trouble breathing, eating, walking, urinating, or defecating
• Multiple lumps appearing quickly or enlarged lymph nodes with illness

When to schedule a vet visit (non-urgent but needed)

• Any new lump that does not resolve within 3–7 days, or any lump present for more than two weeks
• Firm or hard masses, immobile lumps, or lumps that are getting bigger over weeks
• Masses along the mammary chain in intact or previously intact females
• Any lump located where your cat recently received an injection (vaccination or medication)
• Lumps in older cats or cats with weight loss/appetite change

Home care while you wait for your appointment

• Do not lance, squeeze, or try to drain a lump yourself — this can push infection deeper, spread tumor cells (rare but possible with sarcomas), or cause more harm.
• For painful, hot abscesses you can apply a warm compress (3–5 minutes, several times daily) to soothe discomfort while arranging vet care.
• Keep the area clean and prevent your cat from licking or biting it — use an Elizabethan collar if needed.
• If your cat is on any medications (antibiotics, steroids), continue as advised by your vet — do not start new medications without veterinary guidance.
• Provide supportive care: keep the cat warm, encourage eating with palatable food, and ensure hydration.

Diagnostics your vet may recommend

• Fine needle aspirate (FNA) for cytology — quick, often done in-clinic to assess cells from the mass
• Biopsy (incisional or excisional) — provides definitive diagnosis and is especially important if cancer is suspected
• Bloodwork (CBC/chemistry) to look for systemic illness
• Imaging (X-ray, ultrasound) to check for local invasion or metastasis (especially for mammary tumors and sarcomas)
• FIV/FeLV testing if infection or immune suppression is a concern

What to tell your vet (prepare these details)

• Exact location of the lump (be specific: e.g., left rear flank, right axilla, lower abdomen along the second mammary gland)
• When you first noticed it and how it has changed (size, colour, discharge, pain)
• Any recent fights, wounds, surgeries, or injections in that area (dates and products if known)
• Vaccination history and approximate dates
• Spay/neuter status and reproductive history (especially for mammary lumps)
• Appetite, weight changes, energy level, vomiting, diarrhea, coughing, or breathing changes
• Any medications or supplements your cat is currently taking
• Photos and measurements you took at home

Final notes — stay observant, act early

Not every lump is cancer — many are treatable infections or benign masses. However, because cats have a higher proportion of malignant skin and mammary tumors compared with dogs, early veterinary assessment and tissue diagnosis are important. If in doubt, seek veterinary advice rather than waiting. Your vet will guide diagnostics and treatment options based on the lump’s location, appearance, and your cat’s overall health.

References

• Merck Veterinary Manual — Skin disorders and tumors: https://www.merckvetmanual.com
• Cornell University College of Veterinary Medicine — Cornell Feline Health Center: https://www.vet.cornell.edu/college

Frequently Asked Questions