The efficacy of prostaglandin E1 as a pharmacological treatment of erectile dysfunction (impotence) was compared with the standard treatment, papaverine, in a single blind, crossover trial. A total of 129 impotent men received intracavernous injections of either prostaglandin E1 (5 micrograms) or papaverine (18 mg.) 1 month apart. Observations by 1 observer recorded 10 to 20 minutes after injection demonstrated that prostaglandin E1 generated a better erection in 72 men (55.8%) compared to papaverine. Papaverine was more effective than prostaglandin E1 in 23 men (17.8%, chi-square 6.26, p less than 0.025). Subjective assessment by the patients who detected a difference showed that 71 (55%) preferred prostaglandin E1 and 35 (27%) preferred papaverine (chi-square 11.56, p less than 0.001). A total of 34 men achieved full erections with prostaglandin E1 compared to 17 who used papaverine.