Cryptorchidism is common in male dogs, occurring at a rate of up to 10%. This condition is one of the most common congenital defects in purebred dogs (1-11%), with 14% reported in Siberian Huskies. Although the genetics are not fully understood, it is thought to be a recessive, and probably polygenetic, trait. Some have speculated that it is a sex-limited autosomal recessive trait; however, it is unlikely to be simple recessive. Dog testes usually descend by ten days of age and it is considered to be cryptorchidism if they do not descend by the age of eight weeks. Cryptorchidism can be either bilateral (causing sterility) or unilateral, and inguinal or abdominal (or both). Because it is an inherited trait, affected dogs should not be bred and should be castrated. The parents should be considered carriers of the defect and a breeder should thoughtfully consider whether to breed the carrier parent or not. Littermates may be normal, carriers, or cryptorchid. Castration of the undescended teste(s) should be considered for cryptorchid dogs due to the high rate of testicular cancer, especially sertoli cell tumors. The incidence of testicular cancer is 13.6 times higher in dogs with abdominally retained testicles compared with normal dogs. Testicular torsion is also more likely in retained testicles. Surgical correction is by palpation of the retained testicle and subsequent exploration of the inguinal canal or abdomen, however, it is against AKC rules to show altered dogs, making this correction pointless for breeding stock. Surgical correction is termed orchiopexy, i.e., a surgery to move an undescended testicle into the scrotum and permanently fix it there. Surgical correction is an option for pet dogs that will not be used for breeding.

Commonly affected breeds include:

- Alaskan Klee Kai

- Boxer

- Chihuahua

- Dachshund (miniature)

- Bulldog

- Maltese

- Miniature Schnauzer

- Pekingese

- Pomeranian

- Poodle (toy and miniature)

- Pug

- Shetland Sheepdog

- Siberian Husky

- Whippet

- Yorkshire Terrier

Cryptorchidism is rarer in cats than it is in dogs. In one study 1.9% of intact male cats were cryptorchid. Persians are predisposed. Normally the testicles are in the scrotum by the age of six to eight weeks. Male cats with one cryptorchid testicle may still be fertile; however, male cats with two cryptorchid testicles are most likely to be sterile. Urine spraying is one indication that a cat with no observable testicles may not be neutered; other signs are the presence of enlarged jowls, thickened facial and neck skin, and spines on the penis (which usually regress within six weeks after castration). Most cryptorchid cats present with an inguinal testicle.

Testicular tumors and testicular torsion are rare in cryptorchid cats, but castration is usually performed due to unwanted behavior such as urine spraying.