Conventional radionuclide renography has been modified to include a pharmacologic intervention by administration of intravenous furosemide. The procedure is applied in patients with hydroureteronephrosis to distinguish dilated, nonobstructed systems from those with significant mechanical obstruction. Diagnostic patterns are derived from computer-generated time-activity histograms that depict the accumulation of radiotracer prior to diuresis and in response to diuresis. In dilated, nonobstructed systems, increased urine flow following diuresis causes a decline or "washour" of activity. In significantly obstructed systems, there is a failure of tracer activity to decrease in response to diuresis. The procedure is applied most commonly in suspected ureteropelvic junction and ureterovesical junction obstruction. The significance of residual urinary tract dilatation following corrective surgery also can be readily assessed. The major current limitation to the technique is poor renal function with inadequate response of urine flow to diuresis.