Laparoscopic unroofing of a renal cyst was performed in 13 cases of simple cysts of 48 ml. to 678 ml. (mean: 217 ml.) preoperatively measured by ultrasonography from April, 1994 through April, 1995 at our Department of Urology. Under general anesthesia, the renal cyst wall was resected as close as possible to the renal parenchyma by the laparoscopic technique. The postoperative outcome was evaluated in 12 of 13 cases, except for the one case converted to laparotomy because of uncontrollable bleeding from the resected site of the renal parenchyma. Three months after the operation, complete disappearance of the renal cyst was noted by CT scanning in 10 of the 12 cases. In the remaining 2 cases, the renal cyst was still in existence despite the apparent reduction of the cyst volume. In one case in which a somewhat large cyst remained, sclerotherapy using minocycline was carried out. No serious complications during the operation were observed, but in one case with uncontrollable bleeding as mentioned above, the postoperative course was uneventful. These findings indicate that, the laparoscopic unroofing of a renal cyst is a safe and useful procedure for a relatively large renal simple cyst, therefore this approach seems to be acceptable, and before long it will be an ordinary urological operation.