PURPOSE: To evaluate the clinical usefulness of the MR urography (MRU) in the diagnosis of hydroureteronephrosis. MATERIALS AND METHODS: Twenty-five patients with hydronephrosis (26 cases) underwent MR urography, using a 1.5T MR scanner (Signa Horizon, GE Medical Systems, Milwaukee, U.S.A). The causes of hydronephrosis included benign ureteral stricture (12), ureteral stones (8) and malignant strictures (6), and were confirmed by operation, biopsy, conventional urography or clinical follow up. For MRU, a fat suppressed, respiratory-triggered, heavily T2-weighted fast spin echo (FSE) pulse sequence was used, and the 3-dimensional MR images were subsequently processed by a maximum-intensity-projection algorithm. We evaluated the success rate of MRU and its accuracy in determining the level and cause of obstruction, and compared the degree of hydroureteronephrosis seen on MRU with that seen on conventional urography. RESULTS: In all cases, a urogram was successfully obtained. In determining the degree of hydroureteronephrosis and the level of obstruction (in the upper ureter in 11 cases, in the mid-ureter in four, and in the lower ureter in 11), MRU findings matched those of conventional urography. In 24/26 cases (92%), MRU accurately determined the cause of urinary tract obstruction ; the two exceptions were in cases involving ureteral stones. CONCLUSION: MRU is a promising, noninvasive diagnostic method for evaluating hydroureteronephrosis, especially in children, pregnant women and patients with an allergic reaction to iodinated contrast material.