Cryptorchidism is a condition affecting the dog in which one or both testicles do not fully descend into the scrotum. Unilateral cryptorchidism refers to the normal descent of a singular testis.
Bilateral cryptorchidism results in the retention of both testes.
Due to the thermal suppression of spermatogenesis, bilateral cryptorchids are sterile while unilateral cryptorchids are usually fertile. Saint Bernards are noted as a breed at risk. An additional phenomenon, monorchidism, may occur in very few dogs. Monorchidism is a rare condition in which only one testis actually develops in the dog's body. Confirmation of monorchidism may only be determined via plasma testosterone analysis.