The 99mTc-labelled RBC erection penogram (Tc-RBC penogram) coupled with intracavernosal injection of prostaglandin E1 (PGE1) was used to evaluate the penile hemodynamic changes during tumescence. As expected, the method is useful to differentiate organic from psychogenic impotence, and arteriogenic from venogenic impotence. Thirty-four patients with psychogenic impotence, 23 patients with arteriogenic impotence, and 27 patients with venogenic impotence were included in the study. A Tc-RBC penogram was performed on each patient after intracavernosal injection of PGE1. Several parameters calculated from the time-radioactivity curve of the Tc-RBC penogram were used to evaluate hemodynamic changes of the penis. They included (1) the arterial phase: slope and time to peak (TTP), and (2) the venous phase: washout index (WOI). During the arterial phase, the patients with psychogenic and venogenic impotence had higher slope and shorter TTP than those of patients with arteriogenic impotence (p < 0.01). During the venous phase, the patients with psychogenic and arteriogenic impotence had a lower WOI than that of patients with venogenic impotence (p < 0.01). We conclude that the Tc-RBC penogram is a simple, noninvasive, and valuable method for the objective differentiation among psychogenic, arteriogenic, and venogenic impotence.