A standardized technique for percutaneous therapeutic management of renal cysts is described. After partial aspiration of the cyst fluid, the fluid is replaced with Pantopaque, Renografin 60 and air. Fifty-six renal cysts so treated were followed at intervals of up to four years and 68% showed a 50% or more decrease in cyst volume. Only 13% of a control group of 15 renal cysts which were percutaneously punctured but did not have Pantopaque instilled showed a 50% or more decrease in cyst volume. The results suggest that instillation of Pantopaque into a renal cyst can alter the natural history of the cyst by causing it to decrease in size.