Abdominal CT scans of 103 patients without evidence of renal disease were examined. Unsuspected simple renal cysts were demonstrated in 25 (24%). No cysts were demonstrated in patients less than 40 years old. Thereafter the number of cysts and the number of patients with cysts tended to increase with age. Cyst dimensions also increased. These observations suggest that the development of simple cysts of the kidney is related to aging.