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Objective: Electroconvulsive therapy (ECT) is an effective treatment of major depressive disorder (MDD). However, high relapse rates after ECT represent clinical problems. To date, influence of number of ECT sessions on relapse rate remains to be elucidated. We evaluated associations between number of ECT sessions and relapse rate., Methods: This retrospective review collected clinical data of 53 patients with MDD who received ECT. They underwent a 1-year follow-up after their last ECT session. We performed survival analysis to evaluate associations between number of ECT sessions and time until rehospitalisation or suicide., Results: The patients were divided into a higher number of ECT group (≧8 sessions) and lower number of ECT group (<8 sessions). No significant difference was found regarding the patients' clinical and demographic data. Survival analysis using log-rank test revealed that the cumulative survival rate in the higher number of ECT group (79%) was higher compared with the lower number of ECT group (49%) ( p  = 0.042)., Conclusion: Patients who underwent a higher number of ECT had improved survival rate compared with those who received a lower number. Therefore, additional sessions might be necessary, even in patients who achieved remission within seven ECT sessions, to prevent relapse.Key pointsHigh rate of relapse after ECT is a key problem.Impact of the Number of ECT sessions on relapse remains to be elucidated.In the present study, the patients with MDD who underwent eight or more sessions of ECT showed significant lower relapse rate compared with those who received less than eight sessions.