PURPOSE: After the market launch of multiple phosphodiesterase type 5 inhibitors (PDE5Is), a considerable amount of information has emerged regarding the efficacy and the time of initiation of these drugs. In clinical situations, however, many patients with erectile dysfunction (ED) have complained about the onset of erection occurring later than expected or desired. We therefore studied the time course for initiating an erection after usage of PDE5Is. MATERIALS AND METHODS: One hundred forty-one patients who were medicated with PDE5Is > 4 times in the most recent 3 months were included for study. The patients were divided into 3 groups: sildenafil (n=51), vardenafil (n=51), and tadalafil (n=39). The choice of PDE5I was made according to the patient's and/or doctor's preferences. Regardless of the type of drug selected, the patients were recommended to have sexual stimulation 15 minutes after taking the medication. RESULTS: The onset of action of the 3 drug groups were significantly different (sildenafil, 57.0+/-38.5 min; tadalafil, 79.5+/-50.6 min; and vardenafil, 44.4+/-26.6 min; p<0.05). Psychogenic causes of ED resulted in a shorter PDE5I onset of action than organic causes of ED. Other factors, such as body mass index and international index of erectile function erectile function domain score, were shown not to differ with respect to the onset of action of PED5Is. CONCLUSION: In clinical situations, patients need more time for the onset of erections after taking PDE5Is. For restoration of a healthy sexual life, patients needs more time to achieve an erection after taking PDE5Is.