Prazosin HCl was administered to 20 patients with urinary obstructions associated with prostatic hypertrophy. Significant reductions were recorded in the nighttime and the mean 24-hour urination frequencies. Patient-assessed symptoms diminished significantly, and uroflowmetry revealed significant increases in average and maximum urine flow rates. Residual urine volume decreased, the residual urine ratio declined, and bladder compliance tended to increase. The prazosin HCl therapy produced no significant effects on blood pressure or pulse rate, and no side effects appeared. The therapy was judged to be 'extremely beneficial' in 10%, 'beneficial' or better in 50%, and 'marginally beneficial' or better in 85% of all the patients.