Testicular tumors are common in older male dogs that haven't been neutered, and the good news is that most are benign and cured by neutering.1,2,3 The three main types are interstitial (Leydig) cell tumors, seminomas and Sertoli cell tumors.2,1 A retained testicle (cryptorchidism) is about 10 times more likely to develop a tumor, which is why vets recommend neutering dogs with this condition.4,5,3 Some Sertoli cell tumors make estrogen, which can cause enlarged nipples and hair loss and, rarely, dangerous bone marrow suppression.3,2,6 Neutering removes the risk of testicular cancer completely.7,8
See your vet promptly if an unneutered male dog has:
- A lump in a testicle, one testicle larger or firmer than the other, or one that has shrunk1,2
- Enlarged nipples, a hanging sheath, hair loss on both sides of the body or darkened skin, or other males showing interest in him1,2,3
- A testicle that never came down into the scrotum4
Go to a vet right away if he:
What the Research Says
- Tumors are common in older intact males. At one US hospital, 59% of testicles removed and examined had a tumor, in dogs aged 4 to 17.2
- Mixed tumors happen. Some dogs have more than one type of tumor in the same testicle, in about 8% to 17% of cases.2,9
- Malignant behavior is uncommon. In a Brazilian study of 190 dogs with testicular tumors, 6 of 60 seminomas were malignant and only 3 had spread to lymph nodes.10
- Bone marrow suppression is serious but often reversible. In 7 dogs with estrogen-related marrow suppression from a Sertoli cell tumor, all had surgery, 5 needed transfusions and 4 lived more than a year.6
Types of Testicular Tumors
| Type | What it does | Risk of spread |
|---|---|---|
| Interstitial (Leydig) cell tumor | Usually benign; rarely makes hormones2,1 | Very rare3,2 |
| Seminoma | Looks aggressive under the microscope but usually behaves benignly2 | Under about 1 in 103,10 |
| Sertoli cell tumor | The type most likely to make estrogen and cause feminization; common in retained testicles2,5,11 | Under about 1 in 73,1 |
The three types each make up roughly a fifth to a half of cases, depending on the study.2,10,11
Feminization and Bone Marrow Suppression
Some Sertoli cell tumors, up to about half in older reports, make estrogen.3 Signs include enlarged nipples and mammary glands, a hanging sheath, hair loss on both sides of the body, darkened skin, shrinking of the other testicle, an enlarged prostate and attracting male dogs.3,2,1 Feminization is uncommon overall, seen in about 6% of dogs with testicular tumors in one Brazilian study.10
Rarely, high estrogen suppresses the bone marrow, causing anemia and low platelets. These dogs need urgent surgery and often transfusions. Blood counts usually improve within weeks of surgery, but some dogs don't recover.6,3,12
Retained Testicles and Cancer Risk
Somewhere between about 1 and 13 in 100 male dogs have a retained testicle, and small breeds are affected more often.4,5,1 The condition is inherited, so affected dogs shouldn't be bred.5 A retained testicle has about 9 to 14 times the tumor risk of a normal one, mostly Sertoli cell tumors and seminomas.5,4 Vets recommend removing both testicles.4,13,5 See neutering a dog.
Diagnosis
Many tumors are felt as a lump or as one testicle that is larger or firmer than the other. Tumors in a retained testicle may be found as a belly mass on ultrasound.1,4 If feminization signs are present, your vet will check blood counts before surgery.6,3 The type is confirmed by examining the removed testicle.2
Treatment and Outlook
Removing both testicles is the treatment of choice and is usually curative.1,2,3 Feminization signs often fade after surgery.3 For the few tumors that have spread, chemotherapy may be offered, and the outlook is more guarded.3 Most dogs go on to live normally.2,3
Frequently Asked Questions
Is testicular cancer in dogs curable?
Usually, yes. Most testicular tumors are benign and cured by neutering.
What is the life expectancy of a dog with testicular cancer?
Most dogs live normally after neutering. The outlook is more guarded if the tumor has spread or has suppressed the bone marrow.
Does a retained testicle cause cancer?
It raises the risk about 10 times, which is why vets recommend neutering.
What are the first signs of testicular cancer in dogs?
A lump, one testicle larger than the other, or feminization signs such as enlarged nipples and hair loss.
Can neutered dogs get testicular cancer?
No. Neutering removes the testicles and the risk.
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Sources and further reading
This article was rewritten and fact-checked on October 29, 2026 against current veterinary references and specialist guidelines:
- Grannum S. Testicular Tumors in Dogs. PetMD, May 2023.
- Mouser P. Canine testicular neoplasia. MSPCA-Angell Animal Medical Center, April 2025.
- Hoskins JD. Testicular cancer remains easily preventable disease. dvm360, May 1, 2004.
- Weir M, Williams K, Llera R, Downing R, Ward E. Retained Testicle (Cryptorchidism) in Dogs. VCA Animal Hospitals, updated May 2025.
- Memon M, Tibary A. Canine and Feline Cryptorchidism. Recent Advances in Small Animal Reproduction (IVIS), 2001.
- Salyer SA, Lapsley JM, Palm CA, Culp WTN, Lundberg AP, Phillips H, McKenna C, Oblak ML, Hall R, Wustefeld-Janssens B, Tremolada G, Selmic LE. Outcome of dogs with bone marrow suppression secondary to Sertoli cell tumour. Veterinary and Comparative Oncology 2022;20(2):484-490.
- Canadian Veterinary Medical Association. Neutering of Male Dogs. CVMA, October 2012.
- American Veterinary Medical Association. Cancer in pets. AVMA pet owner resource (n.d.).
- (Authors not shown on fetched page.) Histological analysis of multiple unilateral testicular tumors in dogs. Life 2025;15(11):1772.
- Nascimento HHL, dos Santos A, Prante AL, Lamego EC, Tondo LAS, Flores MM, Fighera RA, Kommers GD. Testicular tumors in 190 dogs: clinical, macroscopic and histopathological aspects. Pesquisa Veterinária Brasileira 2020;40(7):525-535.
- Gazin AA, Vatnikov YA, Sturov NV, et al. Canine testicular tumors: an 11-year retrospective study of 358 cases in Moscow Region, Russia. Veterinary World 2022;15(2):483-487.
- Withers SS, Lawson CM, Burton AG, Rebhun RB, Steffey MA. Management of an invasive and metastatic Sertoli cell tumor with associated myelotoxicosis in a dog. Canadian Veterinary Journal 2016;57(3):299-304.
- Davidson AP. Reproductive Disorders of Male Dogs. Merck Veterinary Manual (pet owner version), reviewed June 2018, updated September 2024.
About this article
Last updated : fully rewritten using an MSPCA-Angell pathology review, VCA, PetMD, dvm360, IVIS-hosted material and studies published 2016 to 2025; added the research summary, types table, feminization and bone marrow suppression, retained testicles, key takeaways and FAQ.
This page is for general information and is not a substitute for an exam by your veterinarian.
