The purpose of the study was to characterize the renal lesions of hereditary renal cystadenocarcinomas in the German shepherd dog using computed tomography (CT). Fourteen dogs with renal cystadenocarcinomas and nodular dermatofibrosis, and two unaffected dogs were studied. There were nine dogs with spontaneous disease and seven dogs from a test mating. The characteristic renal CT findings of renal cystadenocarcinomas were bilateral multiple cysts and tumor masses of various sizes. The earliest changes were detected between 4 and 5 years of age and the smallest cysts measured 2-3 mm in diameter. Abdominal CT examination gives a wide field view and excellent anatomic images of the kidneys. It is easy to differentiate between cysts and solid tumors. CT examination is useful for the early detection of renal cystadenocarcinomas and for screening suspected carrier dogs before breeding.