We examined 80 patients with acute obstructive anuria by ultrasound (US). Four of the 80 patients did not have dilatation of the urinary tract. In all four cases, antegrade pyelography guided by real-time US demonstrated urinary tract obstruction after the four patients had experienced 4, 5, 8, and 34 days of anuria, respectively. Diuresis occurred as a result of percutaneous nephrostomy in all four cases. Three of the patients were successfully treated by percutaneous techniques alone. Our findings demonstrate that even a complete and long-term obstruction of the urinary tract does not necessarily induce dilatation in the upper part of the tract. In such cases, only the direct opacification of the urinary tract can help confirm that the obstruction is present. Even in the absence of dilatation, antegrade pyelography guided by real-time US is a possible diagnostic method and can be the first step in the performance of percutaneous nephrostomy.