Summary—The initial slope of the curve relating peak urinary flow rate to voided volume was found to predict an individual's flow rate at bladder volumes of 200 ml or more in normal subjects and Those with symptoms of outflow obstruction. The numerical value of this initial slope was found to decline in a hyperbolic fashion with increasing age. A similar decrease was observed for peak urinary flow rates at bladder volumes between 200 and 300 ml.The use of initial slope as an index of voiding ability would appear to be just as effective in discriminating between normal and obstructed men as a single flow rate determination at a large volume and would be of most benefit in those patients unable to void large volumes of urin