PURPOSE: Sclerotherapy for renal cysts was performed, using 50% acetic acid as new sclerosing agent. Wereport the methods and results of this procedure. MATERIALS AND METHODS: Fifteen patients underwent sclerotherapyfor renal cyst, using 50% acetic acid. Because four patients were lost to follow-up, only 11 of the 15 wereincluded in this study. The renal cysts, including one infected case, were diagnosed by ultrasonograpy (n=10) ormagnetic resonance imaging (n=1). The patient group consisted of four men and seven women (mean age, 59 years;range, 23-77). At first, the cyst was completely aspirated, and 25 volume% of aspirated volume was replaced with50% sterile acetic acid through the drainage catheter. During the following 20 minutes, the patient changedposition, and the acetic acid was then removed from the cyst. Finally, the drainage catheter was removed, aftercleaning the cyst with saline. After treatment of infection by antibiotics and catheter drainage for 7 days,sclerotherapy in the infected case followed the same procedure. In order to observe changes in the size of renalcysts and recurrence, all patients were followed up by ultrasound between 2 and 8 months. We defined response totherapy as follows: complete regression as under 5 volume%, partial regression as 5-50 volume% and no response asmore than 50 volume% of initial cyst volume. RESULTS: No clinically significant complication occured during theprocedures or follow-up periods. All cysts regressed completely during follow-up of 8 months. Complete regressionoccurred as follows : two cysts at 2 months, seven cysts at 4 months, two cysts at 6 months. Two cysts showedresidues at the last follow-up, at 4 and 6 months, respectiivery. The volume of residual cysts decreased to under5 volume% of initial volume, however. Completely regressed cysts did not recurr during follow-up. CONCLUSION: Acetic acid sclerotherapy for renal cysts showed good results, regardless of the dilntion of sclersoing agent withresidual cyst fluid, and no significant complications. the procedure, therefore, appears to provide effectivetherapy for renal cysts.