OBJECTIVE: This open-label study investigated the efficacy of sildenafil citrate at 12 hours postdose among prior treatment responders. PATIENTS AND METHODS: Eight 100-mg doses of sildenafil were provided to 40 eligible patients with erectile dysfunction (ED), who were to record the results of each sexual attempt in a patient diary. Each patient was instructed to make a sexual attempt at 1 and 12 hours postdose on 4 occasions (part 1) and only at 12 hours postdose on 4 subsequent occasions (part 2). RESULTS: Of 40 enrolled patients, 34 (85%) completed the study. In these evaluable patients, 97% and 74% of patients achieved erections that resulted in successful intercourse at 1 hour and 12 hours postdose, respectively. There was a nonsignificant reduction in the percentage of responders at 12 hours during part 1 versus part 2 of this study (71% versus 78%). CONCLUSIONS: This study demonstrates that, in the majority of these patients with ED, sildenafil remains clinically active 12 hours after administration. Larger studies may be necessary to confirm and clarify the therapeutic window of sildenafil.