Twelve cases of cryptorchidism were found in a colony of Miniature Schnauzer purebred and crossbred dogs. At least nine affected dogs were derived from the same sire directly or indirectly. Of 12 affected dogs, five cases were unilateral and seven were bilateral. Eight of the 12 cases were subjected to anatomic study. Fixation of affected organs was by vascular perfusion or immersion. Testes were separated from epididymides and both were weighed. All unilateral retained testicles were on the right side, and right sided bilaterally retained testes were always smaller than their left sided counterparts. With only one exception, ectopic testes were in the abdominal position. Developmentally, the morphologic appearance of the epididymis of abdominal testes was very primitive in bilateral cases but nearly normal in the unilateral cases. Degree of inbreeding was greater for bilateral cases than unilateral cases. High incidence of cryptorchidism in this colony provided good evidence for hereditary nature of the condition in the Miniature Schnauzer dog. Morphologic observations were suggestive of a multiple gene defect.