Cutaneous ureterostomy (CU) is the most simple and safe method of all permanent urinary diversions, but is associated with a risk for stomal obstruction. It is important to appropriately manage hydronephrosis associated with CU. We evaluated the occurrence of stomal obstruction after CU by 99m Tcmercaptoacetyltriglycine (MAG3) diuretic renography three months after surgery. CU was performed after radical cystectomy in 46 patients (90 renal units,RUs) with a minimum follow-up period of 12 months, including 39 men and seven women. The median follow-up period was 102.1 months. The data analyses were performed with half-times to tracer clearance (T1/2) following furosemide administration. The mean T1/2 was 8.45±8.04 minutes. Seventy-five RUs (83.3%) had T1/2 of less than 15 minutes,and 74 (98. 7%) out of 75 RUs had no hydronephrosis. Ten RUs (11.1%) had T1/2 of more than 20 minutes,and all 10 RUs required stent insertions (six RUs) or became atrophic kidneys (four RUs) without the stent insertion. In conclusion,MAG3 diuretic renography was very useful for diagnosing stomal obstruction after CU. T1/2 of less than 15 minutes could be judged as non-obstructive systems,and T1/2 of more than 20 minutes could be judged as obstructed systems. T1/2 between 15 and 20 minutes indicate equivocal studies. Therefore,we recommend the immediate stent insertion in RUs with T1/2 of more than 20 minutes three months after surgery.