Erectile dysfunction is a common complication in patients with diabetes mellitus, which impairs quality of life, decreases self-esteem and can affect partners relationships. Sildenafil improves nitric oxide-dependent relaxation of smooth muscle in corpora cavernosa--induced by an increase in cGMP via inhibition of phosphodiesterase 5. Multicenter, randomized, double-blind, placebo-controlled study with flexible doses of sildenafil. The study was performed in 16 centers and recruited a total of 112 subjects with diabetes mellitus who had erectile dysfunction. At the start and end of the study, the following questionnaires were administered: International Index of Erectile Function (IIEF), Global Efficacy Assessment Question and Quality of Life Questionnaire (Fugl-Meyer). Of the 112 initially recruited patients, 92 received treatment, sildenafil in 44 and placebo in 48. A clear improvement was observed in the capacity to achieve and maintain an erection; 55.3% diabetic patients receiving sildenafil had at least one successful sexual intercourse (15.6% in the placebo group). In addition, significant improvements were seen in other aspects of the sexual activity of treated subjects. Among those treated with sildenafil, 46.3% reported a clear improvement of erections as compared to their baseline conditions (i.e, prior to treatment) vs only 14.9% in the placebo group. The percentage of a successful intercourse clearly increased, from 6 to 49%. Sildenafil was well-tolerated. Side effects were mild and transient. Sildenafil is an effective, safe treatment for erectile dysfunction in diabetic patients.