Objective To assess the cause of tadalafil failure and the feasibility of successfully rechallenging nonresponding patients. Methods A total of 80 consecutive erectile dysfunction ( ED ) patients who claimed poor response to tadalafil were enrolled into the study.A self-administered tadalafil-use questionnaire composed of eight questions was applied to assess how they had used tadalafil.Subjects were given thorough instruction based on individual answers and four doses of tadalafil 20mg.After a 2-week follow-up,end point efficacy of rechallenge was evaluated using the sexual encounter profile (SEP),which was recommended by international advisory panel in 2004. Results A total of 45 subjects had one or more areas of major suboptimal use of tadalafil:21.2％ did not know that sexual stimulation was necessary for tadalafil to work,87.5％ attempted to use tadalafil less than four times,57.5％ took a maximal dose less than 20 mg,and 84％ felt nervous or anxious.Of the 65 patients undergoing tadalafil rechallenge,30 patients answered “yes” to SEP2 and SEP3.The response rate to rechallenge was 46.2％ ( 30/65 ). Conclusions Inappropriate use of tadalafil was major cause of tadalafil non-pesponse.The efficacy of tadalafil could be improved to a better extent by education of patients.