For more accurate determination of the penile erectile volume in the flaccid and erectile states, and the difference between them, 20 impotent patients from 36 years old to 70 years old were enrolled in the study. Penile Doppler ultrasonography was performed to check the diameter of each corpus cavernosum, and diameter and peak flow velocity of each cavernous artery before and after intracavernous injection with 10 pg to 20 pg prostaglandin E l (PGE1). The penile length was measured from base to midglans manually before and after PGEl injection. The erectile volume in flaccid and erectile states was calculated as 7~3 x length. The summation of erectile volume of left and right corpus cavernosum was taken as the total erectile volume. The patients were classified into good (10 patients) and poor (10 patients) response groups based mainly on the mean peak flow velocity of both cavernous arteries greater or less than 25 c d s . There was a significant increase of erectile parameters (penile length, diameter of corpus cavernosum, and total erectile volume) after PGEl injection in each group of patients. For the total of 20 patients, the mean increase of penile length was 4.27 cm (SO.O%), the diameter of corpus cavernosum was 0.66 cm (77.9%), and total erectile volume was 27.87 mL (459.0%). A comparison of the increase of penile erectile parameters after PGEl injection between the good and poor response groups of patients revealed a significantly greater increase of the diameter of corpus cavernosum in the good response patients. Indexing Words: Impotence * Duplex ultrasonography * Pulsed Doppler * Penile erectile volume