OBJECTIVE To compare the effectiveness of intracavernous administration of sodium nitroprusside and prostaglandin E1 to induce penile erection in men with erectile dysfunction. METHODS 100 patients with erectile dysfunction entered the study prospectively. As part of the diagnostic workup, each patient received an intracavernous injection of 20 microg prostaglandin E1 and a second injection of 600 microg sodium nitroprusside 1-7 days later. A tourniquet was placed at the base of the penis before each injection. The data recorded included time required to initiate tumescence, local and systemic side effects, objective and subjective quality of erections, duration of tumescence and patient satisfaction by means of a personal questionnaire. RESULTS Prostaglandin E1 induced better overall responses than sodium nitroprusside, the difference being almost significant (p = 0.055). The overall duration of erections was also significantly longer with prostaglandin E1 (mean 81.3 min) than with sodium nitroprusside (mean 65.4 min; p < 0.04). 67% of the patients considered the erections induced with prostaglandin E1 to be of better quality than those with sodium nitroprusside, and only 11% stated that sodium nitroprusside was superior. Side effects were minimal with both drugs, the most frequent side effect being systemic hypotension, which was induced by sodium nitroprusside in 7% of the patients. CONCLUSIONS The moderate risk of systemic hypotension and the lower potency of sodium nitroprusside to induce erections compared to prostaglandin E1 rules out sodium nitroprusside as a routine alternative intracavernous drug in men with erectile dysfunction at the doses employed. Sodium nitroprusside, however, could be used in patients who have intolerance or penile pain with intracavernous prostaglandin E1.