Testicular tumors are most common in intact (unneutered) older male dogs. However, they can occur in intact males of any age. There does not appear to be any breed predilection for this tumor. The current cause of testicular tumors is unknown. Dogs that have one or both testicles that are not descended (cryptorchid) are 13 times more likely to develop a tumor in the undescended testicle than dogs with normal testicles. Except for the increased risk of these tumors in cryptorchid dogs, no other risk factors are readily apparent.