Cancer in dogs often starts with subtle signs that are easy to put down to "just getting older": a new lump, weight loss, a sore that won't heal or a dog that slows down on walks.1,2 Many of these signs have other causes too, but because about 1 in 4 dogs develop cancer, and about half of dogs aged 10 or older, it's worth getting any persistent change checked.2 Finding cancer early usually means more treatment options and a better outlook.3 This guide covers the warning signs, what lumps to worry about, signs linked to specific cancers, and when to see a vet.
Go to an emergency vet now if your dog:
- Suddenly collapses or becomes very weak4
- Has pale or white gums, or a belly that suddenly swells4,5,6
- Is breathing fast or struggling to breathe4
These can be signs of internal bleeding from a ruptured tumor, such as hemangiosarcoma.4
What the Research Shows
- The 1 cm / 1 month rule. Veterinary oncologist Dr. Sue Ettinger's "See Something, Do Something" campaign advises sampling any lump that is pea-sized (1 cm) or larger and has been present for a month.3,7
- Skin lumps are common, and some are cancer. Skin samples make up 25% to 43% of biopsies from dogs and cats, and 20% to 40% of those are malignant.3
- Swollen lymph nodes point to lymphoma. Painless, swollen lymph nodes are seen in 80% to 85% of dogs with the common forms of lymphoma.8
- Thirst can be a clue. 10% to 40% of dogs with lymphoma have high blood calcium, which causes increased thirst and urination.8
- Mast cell tumors are common. They make up 11% to 20% of skin tumors in dogs.9
10 Warning Signs of Cancer in Dogs
The Veterinary Cancer Society lists these common warning signs:1
- Abnormal swellings that persist or continue to grow
- Sores that don't heal
- Weight loss
- Loss of appetite
- Bleeding or discharge from any body opening
- Offensive odor
- Difficulty eating or swallowing
- Hesitation to exercise or loss of stamina
- Persistent lameness or stiffness
- Difficulty breathing, urinating or defecating
The AVMA adds abdominal swelling, persistent vomiting or diarrhea, lumps that change color or texture, and behavior changes such as hiding or losing interest in activities.2
Lumps and Bumps: When to Worry
Many lumps are benign, such as fatty lipomas, but in studies of skin samples sent to labs 20% to 40% were malignant, and you can't tell which by looking or feeling.3 Specialists recommend the "See something, do something" approach: if a lump is pea-sized (1 cm) or larger and has been there for a month, ask your vet to sample it with a needle aspirate or biopsy.3,7 Measure lumps and take photos so you can track changes. Lumps that grow quickly, change, ulcerate or bleed should be checked sooner.2,1
Signs of Specific Cancers
- Lymphoma: swollen, painless lymph nodes in the neck, armpits, groin or behind the knees; sometimes increased thirst, weight loss or lethargy.10,11,8 See lymphoma in dogs.
- Hemangiosarcoma: often no signs until sudden weakness, collapse, pale gums or a swollen belly from internal bleeding.4 See hemangiosarcoma in dogs.
- Bone cancer (osteosarcoma): lameness or a painful swelling on a leg, usually in large breeds.9 See bone cancer in dogs.
- Mouth cancer: bad breath, drooling, bleeding from the mouth, a mass on the gums or difficulty eating.1,9 See signs of oral cancer.
- Skin cancers, including mast cell tumors: lumps in or under the skin that can suddenly change in size.9 See mast cell tumors.
My Dog Has Cancer but Acts Fine: Is That Normal?
Yes. Many dogs feel well when cancer is diagnosed,10 and tumors found and treated early often have a better outlook.3 Keep an eye on appetite, energy, breathing and weight, and tell your vet about any change.
What Happens at the Vet
Your vet will examine your dog, feel the lymph nodes and any lumps, and usually take a needle sample or biopsy. Blood tests, X-rays and ultrasound help find out whether a cancer has spread.3,12 For an overview of types and treatments, see cancer in dogs.
Frequently Asked Questions
What are the first signs of cancer in dogs?
New or growing lumps, sores that won't heal, weight loss, poor appetite, bleeding or discharge, lameness and reduced energy.
How will my dog act if he has cancer?
Some dogs act normally; others become tired, eat less, lose weight, hide or lose interest in play.
Can a vet tell if a lump is cancer by feeling it?
No. A needle aspirate or biopsy is needed to know what a lump is.
Should every lump on my dog be checked?
Any lump that is pea-sized (1 cm) or larger and has been there a month should be sampled by a vet, and any lump that is growing or changing should be checked.
Does cancer show up in blood work?
Sometimes, but not always. Many cancers need imaging, a needle sample or a biopsy to diagnose.
What is the "silent" cancer in dogs?
Hemangiosarcoma is often called silent because it may cause no signs until it bleeds internally.
At what age do dogs get cancer?
It's most common in older dogs, but some cancers, such as bone cancer, have a second peak in dogs aged 1 to 2.
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Sources and further reading
This article was rewritten and fact-checked on October 16, 2026 against current veterinary references and specialist guidelines:
- Tufts University / Your Dog newsletter. 10 Warning Signs of Cancer (lists the Veterinary Cancer Society's 10 signs; n.d.).
- American Veterinary Medical Association. Cancer in pets. AVMA pet owner resource (n.d.).
- Ettinger SN. Tumor Detection in Dogs and Cats: See Something, Do Something. Today's Veterinary Practice, Jul/Aug 2017.
- Cornell Richard P. Riney Canine Health Center. Hemangiosarcoma in dogs (n.d.).
- American College of Veterinary Surgeons. Splenic Masses (n.d.).
- Phillips B, Flory A. Hemangiosarcoma cancer in dogs and cats. VetSpecialists.com (board-certified specialists), 2020.
- Ettinger SN. See something, do something: Why wait? Aspirate. dvm360, 8 Jun 2015.
- Fan TM. Overview of Canine Lymphoma. Merck Veterinary Manual, Professional Version. Reviewed Sep 2023, modified Apr 2025.
- National Cancer Institute, Center for Cancer Research, Comparative Oncology Program. Disease info (n.d.).
- Hyland K, Weir M, Barnette C. Lymphoma in Dogs. VCA Animal Hospitals (n.d.).
- Merck Veterinary Manual, Pet Owner Version. Malignant Lymphoma in Dogs.
- Cornell University Hospital for Animals. Cancer management: frequently asked questions (n.d.).
About this article
Last updated : fully rewritten using Veterinary Cancer Society, AVMA, Merck Veterinary Manual and veterinary oncology guidance on early warning signs and lump checks; added a research summary and FAQ.
This page is for general information and is not a substitute for an exam by your veterinarian.
