A lump on your cat’s side could be anything from a harmless cyst to an abscess, a fatty deposit, or a tumor that needs treatment. Skin and subcutaneous tumors are the most common type of tumor in cats overall, but not all lumps are tumors, and not all tumors are cancer. In a large study of over 400 skin and subcutaneous masses in cats, about two-thirds turned out to be benign. Still, one in three was malignant, which is why any new lump deserves a veterinary evaluation rather than a wait-and-see approach.
Abscesses From Bites or Wounds
If your cat goes outdoors or lives with other animals, one of the most likely explanations is an abscess. An abscess forms when bacteria get trapped under the skin after a bite or scratch heals over on the surface, sealing infection underneath. The result is a warm, painful lump that may grow quickly over a few days. You might notice thin yellow fluid or thick white pus leaking from it, and your cat may flinch or hiss when you touch the area. Abscesses on the side and flank are common because cats tend to get bitten along their body during fights.
Most abscesses need to be drained and flushed by a vet, and your cat will likely get a course of antibiotics. Left untreated, the infection can spread deeper or rupture on its own, leaving a messy open wound that heals slowly.
Cysts
Cat cysts are benign masses that contain fluid or thicker, non-infected material. Some are filled with keratin, a protein naturally found in skin. They typically feel round or oval and can be either firm or soft. Unlike abscesses, cysts usually aren’t painful and don’t feel warm. They tend to grow slowly, if at all, and your cat probably won’t react when you touch one.
Most cysts don’t need treatment unless they become infected, rupture, or grow large enough to bother your cat. Your vet can usually tell a cyst from something more concerning with a simple needle sample.
Lipomas (Fatty Lumps)
Lipomas are benign growths made of fat cells. They feel rubbery and relatively soft, and they tend to move freely under the skin when you push on them. Lipomas are more common in dogs than cats, but cats do get them. If your cat is overweight, any lipoma will tend to be larger and more noticeable. Cats that lose weight will see their lipomas shrink, though they rarely disappear entirely.
The tricky part is that a few types of malignant tumors can mimic the rubbery texture of a lipoma. Soft tissue sarcomas, for instance, can feel similar, though they tend to feel more deeply rooted and firmer. This is one reason vets recommend testing rather than assuming a soft lump is harmless. There’s also an infiltrative version of the lipoma that grows into surrounding tissue, making it harder to remove, though it doesn’t spread to distant parts of the body.
Mast Cell Tumors
Mast cell tumors are one of the more common skin cancers in cats, and the trunk (your cat’s side) is one of the places they appear most often, along with the head and neck. They can look like small, flesh-colored, hairless nodules. Some cats develop just one; others develop several at once. Mast cell tumors can also occur internally, particularly in the spleen and intestinal tract, so a skin lump that turns out to be a mast cell tumor may prompt your vet to check for internal involvement with imaging.
Not all mast cell tumors behave aggressively. Some cutaneous forms carry a good prognosis after surgical removal. But because their behavior varies, getting a diagnosis early gives your cat the best range of treatment options.
Post-Vaccination Lumps
If your cat was recently vaccinated, a small, firm swelling at the injection site is a normal inflammatory response. Vaccines are commonly given in the hind legs or along the side, so this is a plausible explanation if the timing lines up. These lumps typically disappear within a couple of weeks.
Rarely, a more serious growth called an injection-site sarcoma can develop where a vaccine was given. Veterinary guidelines use what’s called the 3-2-1 rule to flag lumps that need a biopsy: any mass that remains present three months after vaccination, is larger than 2 centimeters in diameter, or is increasing in size one month after vaccination warrants further testing. If your cat’s post-vaccine lump fits any of those criteria, bring it to your vet’s attention promptly.
How Vets Figure Out What a Lump Is
You can’t reliably diagnose a lump by how it looks or feels alone. Even experienced vets are often surprised by test results. The standard first step is a fine needle aspirate: your vet inserts a small needle into the lump, draws out a sample of cells, and examines them under a microscope. It’s quick, minimally painful for your cat (similar to a blood draw), and doesn’t usually require sedation. In many cases, this single test is enough to distinguish a cyst, lipoma, abscess, or tumor.
If the needle sample is inconclusive, the next step is usually a biopsy, where a larger tissue sample is sent to a pathology lab. For lumps that appear deep or are in an unusual location, your vet might recommend an ultrasound to get a better picture of the mass before deciding on next steps.
Signs That Need Prompt Attention
Some lumps can wait for a scheduled appointment. Others shouldn’t. Contact your vet sooner rather than later if you notice any of the following:
- Rapid growth: a lump that’s noticeably bigger within days or weeks
- Pain or sensitivity: your cat flinches, growls, or avoids being touched near the lump
- Bleeding, discharge, or ulceration: the skin over the lump breaks open or oozes
- Systemic changes: your cat is eating less, losing weight, vomiting, or unusually lethargic
- Location near the throat, eyes, or joints: lumps in these areas can interfere with breathing, vision, or movement as they grow
These signs can point to an abscess that’s worsening, an infected cyst, or a cancerous mass. Even if none of these red flags apply, any new lump that persists for more than two to three weeks is worth having checked. A simple needle test can give you a clear answer and, in most cases, real peace of mind.