Using the previously demonstrated statistical relationship between urinary flow rate and bladder volume we tested 53 patients with clinically evident outflow obstruction by uroflowmetry. Of these patients 32 were tested before and after a successful operation for outflow obstruction. In 98 per cent of the maximum flow measurements in obstructed patients the nomogram values were less than minus 2 standard deviations, indicating a clear separation from the normal population. After operative relief of the obstruction repeat uroflowmetry demonstrated all values to be in the normal range. Furthermore, in each patient the peak flow rate value increased an average of 2.7 standard deviations (minimum increase of 1.5 standard deviations). No significant correlation could be demonstrated between the size of the prostatic resection and the degree of urinary flow impairment. The use of a flow rate nomogram appears to differentiate reliably normal from obstructed individuals and also is highly useful in the postoperative followup of urinary outflow obstruction.