Premature ejaculation (PE) is thought to be the most common male sexual dysfunction; however, the prevalence of lifelong (LL)-PE is relatively low. The aim of this study was to investigate the effects of on-demand vardenafil (10 mg) to modify the intravaginal ejaculatory latency time (IELT) in men with LL-PE without erectile dysfunction. Forty-two men (18-35 years) were enrolled in a 16-week, double-blind, placebo-controlled, cross-over study. Primary end point was the modification from baseline of IELT assessed by stopwatch technique; secondary end points were post-ejaculatory refractory time (PERT) and variations of scores at the Index of Premature Ejaculation questionnaire. The changes in geometric mean IELT were superior after taking vardenafil (0.6+/-0.3 vs 4.5+/-1.1 min, P<0.01), compared with placebo (0.7+/-0.3 vs 0.9+/-1.0 min, ns). PERT dropped significantly after vardenafil (16.7+/-2.0 vs 4.3+/-0.9 min, P<0.001), compared with placebo (15.3+/-2.2 vs 15.8+/-2.3 min). Patients who took vardenafil (vs placebo) reported significantly (P<0.01) increased ejaculatory control (6+/-2 vs 16+/-2), improved overall sexual satisfaction (7+/-2 vs 15+/-1) and distress (4+/-1 vs 8+/-1) scores, respectively. Multiple regression analysis (r(2)=0.86) for IELT by the number of attempts at sexual intercourse showed significant differences between the slopes of lines for placebo and vardenafil (P<0.0001). The most common adverse events for vardenafil (vs placebo) were headache (10 vs 3%), flushing (12 vs 0%) and dyspepsia (10 vs 0%), which tended to disappear over the time. In conclusion, in our study, vardenafil increased IELT and reduced PERT in men with LL-PE. Besides, improvements in confidence, perception of ejaculatory control and overall sexual satisfaction were reported.