The efficacy and safety of oral sildenafil, a potent inhibitor of phosphodiesterase type 5, were evaluated in men with diabetes mellitus and erectile dysfunction (ED). Twenty-one men (aged 42-65 years) were enrolled in a double-blind, placebo-controlled, three-way crossover study conducted in two parts. In part I, the effect of a single dose (25 mg or 50 mg) of sildenafil or placebo on penile rigidity was assessed by penile plethysmography during visual sexual stimulation. In part II, daily diary records of erectile activity and a global efficacy question were used to evaluate once-daily dosing with 25 mg or 50 mg of sildenafil or placebo for 10 days. After a single 50 mg dose of sildenafil, the adjusted geometric mean duration (min) of penile rigidity >60% at the base of the penis during visual sexual stimulation was significantly increased (10.1 min) compared with placebo (2.8 min; p = 0.0053). In part II, sildenafil significantly increased the number of erections considered sufficiently hard for vaginal penetration compared with placebo (p = 0.0005). Improved erections were reported by 50% and 52% of patients treated with 25 mg and 50 mg of sildenafil, respectively, compared with 10% of those receiving placebo (p values < 0.05). Adverse events were mostly mild or moderate in nature and included muscular pains, headache, and dyspepsia. Sildenafil is a well-tolerated and potentially efficacious oral treatment for ED in men with diabetes mellitus.