A 10-year-old boy with a chief complaint of nocturnal enuresis was identified as having three cysts in each kidney on ultrasonographic examination. These findings together with a positive family history for renal failure prompted us to examine his father ultrasonographically on the same day, resulting in the identification of multiple large cysts in both kidneys. These observations supported a diagnosis of autosomal dominant polycystic kidney disease. Enhanced X-ray CT demonstrated more sensitively the presence of renal cysts in the boy. He had a normal urinary concentrating ability and no other abnormalities except for mild hypertension, which was successfully treated only by salt restriction. Ultrasonography thus appears to be a valuable tool for diagnosing polycystic kidney diseases.