Considered the most common congenital testicular abnormality of companion animals and a predisposition factor to the development of testicular neoplasia, cryptorchidism is defined as the non-descent of one or both testes to their normal anatomical location. Data on the occurrence of cryptorchidism in Brazil are scarce. The purpose of this work was to verify the occurrence of cryptorchidism in dogs of different sizes and breeds. Cryptorchidism identification was carried out by ultrasound scanning, from November, 1994 to March, 2007, at the Center for Veterinarian Diagnosis and Support (Centro de Apoio e Diagnóstico Veterinário--CAD), in Rio de Janeiro. 4,924 male dogs of different breeds were examined, revealing 403 (8.2%) cryptorchidism. In this study, occurrence took place more often on the right testicle (59.5%), more frequently displaying inguinal localization (59.5%) and unilateral occurrence (70%). Regarding bilateral presentation, the symmetrical form was the most common (86.8%). Cryptorchidism was more common in the inguinal region of dog of small sized breeds and in the abdominal region in dogs of medium and large sized breeds. Ultrasound scan proved a valuable diagnosis tool for cryptorchid testes, giving precise localization and parenchymal changes thus leading to a safe clinical treatment.