Twenty patients with hydronephrosis in whom examination with ultrasound was helpful in defining the extent of dilation of the collecting system are presented. Thirteen patients had hydronephrosis due to obstruction at the ureteropelvic junction, six had obstruction of the ureterovesical junction from various causes, and one had massive bilateral vesicoureteral reflux. The diagnosis of high ureteral obstruction as opposed to obstruction or reflux at the ureterovesical junction was based on visualizing a dilated ureter in the later two groups.