Cryptorchidism is a condition in which either one or both testes (testicles) fails to descend fully into the scrotum. Normally, the testicles will descend before birth. In young puppies (under 6 months) the testicles can occasionally retract back into the groin, particularly if the pup is cold or excited. There should, however, be periods of time when both testicles are properly located.
     Cryptorchidism presents itself in one of two forms: 1) unilateral cryptorchidism - normal descent of only one testicle, 2) bilateral cryptorchidism - retention of both testicles. Unilateral cryptorchid dogs are usually capable of breeding, whereas bilateral cryptorchids are sterile. It appears that cryptorchidism is polygenetic (involves multiple genes), and therefore elimination of this condition from a gene pool is difficult. This condition occurs more often in some breeds of dog (Saint Bernards and German Shepherd Dogs) than in others, but can and does occur in all dog breeds. Cryptorchidism achieves its highest levels of occurrence in heavily line-bred animals.
     In spite of the fact that bilateral cryptorchid dogs are sterile, both bilateral and unilateral cryptorcids should be neutered, to reduce the risk of possible future complications. Unilateral cryptorchids should never be used in a breeding program.