Background: No drugs are approved for treatment of premature ejaculation. Our aim was to determine the efficacy and tolerability of on-demand dapoxetine in patients with severe premature ejaculation. Methods: We determined the efficacy of dapoxetine in a prospectively predefined integrated analysis of two 12-week randomised, double-blind, placebo-controlled, phase III trials of identical design done independently, in parallel, at 121 sites in the USA. Men with moderate-to-severe premature ejaculation in stable, heterosexual relationships took placebo (n=870), 30 mg dapoxetine (874), or 60 mg dapoxetine (870) on-demand (as needed, 1-3 h before anticipated sexual activity). The primary endpoint was intravaginal ejaculatory latency time (IELT) measured by stopwatch. Safety and tolerability were assessed. All analyses were done on an intention-to-treat basis. The trials are registered at ClinicalTrials.gov, numbers NCT00211107 and NCT00211094. Findings: 672, 676, and 610 patients completed in the placebo, 30 mg dapoxetine, and 60 mg dapoxetine groups, respectively. Dapoxetine significantly prolonged IELT (p<0·0001, all doses vs placebo). Mean IELT at baseline was 0·90 (SD 0·47) minute, 0·92 (0·50) minute, and 0·91 (0·48) minute, and at study endpoint (week 12 or final visit) was 1·75 (2·21) minutes for placebo, 2·78 (3·48) minutes for 30 mg dapoxetine, and 3·32 (3·68) minutes for 60 mg dapoxetine. Both dapoxetine doses were effective on the first dose. Common adverse events (30 mg and 60 mg dapoxetine, respectively) were nausea (8·7%, 20·1%), diarrhoea (3·9%, 6·8%), headache (5·9%, 6·8%), and dizziness (3·0%, 6·2%). Interpretation: On-demand dapoxetine is an effective and generally well tolerated treatment for men with moderate-to-severe premature ejaculation.