The efficacy and safety of tadalafil, dosed once a day for the treatment of erectile dysfunction, was assessed in a randomized, double-blind, placebo-controlled, parallel-design study at 15 US centers. Following a 4-week treatment-free run-in period, patients (>or=18 years of age) were randomly assigned to 24 weeks treatment with tadalafil 2.5 mg, tadalafil 5 mg or placebo. Primary efficacy endpoints were change at 24 weeks in International Index of Erectile Function Erectile Function (EF) Domain score and mean per-patient percentage 'yes' responses to Sexual Encounter Profile diary questions 2 and 3. Tadalafil significantly improved erectile function compared with placebo for all three co-primary efficacy endpoints. Few patients discontinued because of adverse events (2.1%, placebo; 6.3%, tadalafil 2.5 mg; 4.1%, tadalafil 5 mg). Common treatment-emergent adverse events (>or=5%) were nasopharyngitis, influenza, viral gastroenteritis and back pain. Tadalafil 2.5 mg and 5 mg, dosed once a day for 24 weeks, was well tolerated and significantly improved erectile function.