The pharmacodynamic effect on penile rigidity and tumescence and the pharmacokinetic properties of single oral doses of 10 and 20 mg vardenafil, a new PDE5-inhibitor, were investigated in 21 erectile dysfunction patients. Patients were evaluated with RigiScan on three occasions in a randomized, placebo-controlled, double-blind crossover fashion, while receiving visual sexual stimulation. Relative to placebo, a single dose of 10 mg vardenafil led to a mean increase in the duration of >60% penile rigidity of 24.4 min (95% CI: 7.4 to 41.3) at the base and of 24.8 min (8.5 to 41.1) at the tip. For the 20-mg dose, the increase in duration of >60% penile rigidity relative to placebo was 37.2 min (20.2 to 54.1) at the base and 28.7 min (12.7 to 44.7) at the tip. Single doses of 10 and 20 mg vardenafil led to a rapid rise in the plasma concentrations of vardenafil, with a median t_max of 0.9 h and 0.7 h and a geometric mean C_max of 9.1 μg/l (geometric SD=1.63) and 20.9 μg/l (geometric SD=1.83), respectively. In the post-absorptive phase, the concentrations declined with an average terminal t_1/2 of 4.2 h (geometric SD=1.27) and 3.9 h (geometric SD=1.31). The systemic exposure of vardenafil expressed as AUC normalized for dose and body weight was dose-proportional (associated 90% CI: −4 to 30%) as well as C_max (associated 90% CI: −12 to 33%). The treatments were well tolerated. There was a small, clinically irrelevant reduction in blood pressure with a small compensatory rise in heart rate. There were no electrocardiographic effects or relevant changes of the safety laboratory screens. The observed pro-erectile properties, pharmacokinetic characteristics and safety profile make vardenafil a suitable candidate for further evaluation in the treatment of erectile dysfunction.