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Introduction. The efficacy of phosphodiesterase-5 inhibitors in the treatment of erectile dysfunction may depend on patient characteristics. Aim. To determine whether patient characteristics influence the efficacy of two tadalafil dosage regimens and to identify prognostic factors predictive of tadalafil efficacy. Methods. This was a multicenter, open-label study in which men with erectile dysfunction were randomized to tadalafil 20 mg either on demand or three times per week for a period of 5–6 weeks. After a 1-week washout period, patients were crossed over to the alternate regimen for another 5–6 weeks. Main Outcome Measures. Score of the Erectile Function (EF) domain of the International Index of Erectile Function Questionnaire (IIEF) and percentage of positive responses to questions 3 and 5 of the Sexual Encounter Profile (SEP) diary. Results. A total of 4,262 patients were randomized. A normal EF domain score (≥26) at the end of on-demand and three-times-per-week treatment was reported by 60.2% and 62.3% of patients, respectively. The percentage of patients who achieved a normal EF domain score and the percentages reporting positive responses to SEP3 and SEP5 depended on the severity of erectile dysfunction and the presence of certain comorbidities, irrespective of the tadalafil dosage regimen. On regression analysis, the two best predictors of tadalafil efficacy were the baseline score of the IIEF-EF domain and the baseline percentage of “Yes” responses to SEP2. Conclusions. On-demand and three-times-per-week dosage regimens of tadalafil 20 mg were equally efficacious in men with erectile dysfunction. Among the possible prognostic factors tested in this study, baseline disease severity scores were the strongest predictors of efficacy endpoint scores. Costa P, Buvat J, Holmes S, Weitckus S, Petto H, Hamidi K, and Varanese L. Predictors of tadalafil efficacy in men with erectile dysfunction: The SURE study comparing two dosing regimens. J Sex Med 2006;3:1050–1058. [ABSTRACT FROM AUTHOR]