Objective. The aim of the study is to define predictors of relapse/recurrence after electroconvulsive therapy, ECT, for patients with major depressive disorder. Methods. A study of all patients (n = 486) treated by means of ECT for major depressive disorder was performed. The data were derived from a regional quality register in Sweden. Psychiatric hospitalisation or suicide was used as a marker for relapse/recurrence. Results. The relapse/recurrence rate within one year after ECT was 34%. Factors associated with increased risk of relapse/recurrence included comorbid substance dependence and treatment with benzodiazepines or antipsychotics during the follow-up period. Conclusions. Within the first years after ECT, relapses/recurrences leading to hospitalisation or suicide are common. Treatment with lithium might be beneficial, while benzodiazepines, antipsychotics, or continuation ECT does not seem to significantly reduce the risk of relapse/recurrence.