Background and purpose: Several adverse effects were reported for oral sildenafil. In this research after formulation of sildenafil topical gel, the effect of gel was compared with sildenafil tablet in a double-blind placebo-controled clinical trial. Materials and methods: After choosing the solvent system, several formulations were prepared and the most suitable gel was chosen for clinical trial. The study was a controlled randomized (block-random sampling), double blind, prospective, placebo-controlled trial. 94 patients, with clinically diagnosed erectile Dysfunction were recruited. This includes men with ED of organic, psychogenic, and mixed causes. Patients were divided into two categories of age under and over 50. Six blocks were considered for including patients based on the nature of ED and age. All of the patients with ED from July 2003 to May 2004 diagnosed by one urologist were included in the study. The cases received 1% topical gel of sildenafil and placebo tablet, and the control group received sildenafil tablet (100 mg) and gel base (without drug) as placebo. The tablets were taken one hour before sexual action and the topical gel was used in 0.5g (approximately) on the glans of penis and were massaged for 5min, before the sexual activity. Results: In the case group that topical gel of sildenafil was administered, 5 patients (12.5%) had complete erection, 5 patients had moderate erection and erection was not observed in 30 (75%) of them. In control group, that sildenafil tablet was administered, these results were 28 (70%), 6 (15%) and 6 (15%) respectively. The onset of erection in case group (in-patients with complete erection) was 7.4 ± 3.6 min, but this time was 37.8 ± 14.9 min in control group. Four cases of mild headache were observed in-patients who administered topical gel of sildenafil. This was pain treated before 4 min. Two cases of severe headache were observed in-patients who were administered sildenafil tablet. The disorder on visual function was observed in one patient in control group. One case of severe dyspepsia was observed in this group too. Conclusion: Sildenafil delivery using transdermal formulation can be enhanced by several synthetic or natural percutaneous absorption enhancers, and appears to be a promising approach for the treatment of erectile Dysfunction.