In two multicenter, placebo controlled, phase 2 studies, patients with mild-to-moderate (n=161, Study 1) or severe (n=142, Study 2) erectile dysfunction (ED) were randomized to receive placebo, 0.05, 0.1, or 0.2 mg (Study 1) or placebo, 0.1, 0.2, or 0.3 mg (Study 2) of topically applied alprostadil (containing a proprietary skin permeation enhancer). The primary efficacy end point in both studies was the change in erectile function (EF) score from baseline to final visit. The changes from baseline for EF scores were -0.8+/-1.1, 1.8+/-1.1, 0.7+/-1.2, and 3.7+/-1.2 (P<0.01; Study 1) and 2.7+/-1.3, 6.29+/-1.4, 6.49+/-1.5, and 9.44+/-1.5 (P<0.001; Study 2) for ascending dose groups in each study. Topical alprostadil was well tolerated with the most common adverse event being urogenital pain. These results suggest this topical alprostadil formulation may be a potentially useful agent for the treatment of ED.