Under ultrasound guidance, we treated 25 cases of renal cyst with 99% ethanol instillation to prevent the recurrence of this disease from January 1985 to June 1987. Patients' age was from 17 to 85 years old with the average age of 63 years. Twelve cases were men, and 13 cases were women. Among the 25 cases, eleven were asymptomatic and 14 showed clinical features of lumbago, microhematuria, hypertension or proteinuria. The aspirated site was the right side in 9, left side in 14 and bilateral kidneys in 2 cases. Subsequently, cyst puncture was carried out 27 times. We encountered 12 complications following puncture. These complications were derived from the puncture itself or caused by the ethanol instillation. Flank pain caused by the injection of ethanol, nausea, causalgia or a feeling of drunkenness appeared immediately after the inoculation procedure. However, no serious complications such as pneumothorax, perirenal hematoma or infection were recognized. Some complications arose in 7 cases of 9 examples (77.8%) following more than 50 ml of ethanol injection, but the complications were observed in only 5 cases of 18 examples (22.8%) following less than 50 ml of administration. Based on these findings, ethanol injection in renal cysts appears to be useful for the treatment of this disease. In case of huge cysts when more than 50 ml of ethanol, is instilled the case should be followed up carefully after the instillation procedure.