INTRODUCTION: Dilation of urinary tract occurs without the presence of obstruction. Diagnostic methods that depend on renal function may elicit mistaken diagnosis. Whitaker (1973) proposed the evaluation of urinary tract pressure submitted to constant flow. Other investigators proposed perfusion of renal pelvis under controlled pressure, making the method more physiological and reproducible. The objective of the present study was to evaluate the results of the anterograde pressure measurement (APM) of the urinary tract of children with persistent hydronephrosis after surgery suspected to present persistent obstruction. MATERIALS AND METHODS: Along 12 years, 26 renal units with persistent hydronephrosis after surgery (12 PUJ and 14 VUJ) were submitted to evaluation of the renal tract pressure in order to decide the form of treatment. Previous radionuclide scans with DTPA, intravenous pyelographies and ultrasounds were considered undetermined in relation to obstruction in 10 occasions and obstructive in 16. APM was performed under radioscopy through renal pelvis puncture or previous stoma. Saline with methylene blue + iodine contrast was infused under constant pressure of 40 cm H2O to fill the urinary system. The ureteral opening pressure was measured following the opening of the system and stabilization of the water column. RESULTS: Among the 10 cases with undetermined previous diagnosis, APM was considered non-obstructive in two and those were treated clinically and eight were considered obstructive and were submitted to surgery. Among the 16 cases previously classified as obstructive, nine confirmed obstruction and were submitted to surgery. Seven cases were considered non-obstructive, and were treated clinically, with stable DMSA and hydronephrosis. CONCLUSIONS: APM avoided unnecessary surgery in one third of the cases and was important to treatment decision in 100%. We believe that this simple test is an excellent diagnostic tool when selectively applied mainly in the presence of functional deficit.