The durability of key efficacy response parameters and safety of vardenafil was evaluated in a pivotal trial conducted in a broad population of men with erectile dysfunction (ED) in North America. In this randomized, double-blind, placebo-controlled, multicenter, fixed-dose, parallel-group, 6-month comparison study, men >18 years of age with ED for >6 months received 5-mg, 10-mg, and 20-mg doses of vardenafil as needed for up to 26 weeks. The primary efficacy variables were the International Index of Erectile Function (IIEF)-Erectile Function (EF) domain scores, and the Sexual Encounter Profile (SEP) mean per-patient success rates for penetration (SEP question 2) and maintenance of erections (SEP question 3). Safety data were also collected over time. Improvement in all primary efficacy variables was observed in all vardenafil groups versus placebo. These improvements occurred early and were either sustained or increased through week 26. Vardenafil in 10-mg and 20-mg doses was significantly superior to placebo at all time points for all efficacy variables (P <0.01), and all doses were superior to placebo at endpoint (P <0.001). Most treatment-emergent adverse events (headache, flushing, dyspepsia, and rhinitis) were mild or moderate in intensity, and incidence generally decreased over time. All 3 doses of vardenafil were superior to placebo across all primary efficacy variables and all study time points in a broad range of patients with ED, regardless of etiology or severity. Vardenafil was well tolerated. These results demonstrate that vardenafil provides sustained efficacy with reduced incidence of nuisance side effects over time. High resolution video, medium resolution video, low resolution video.