OBJECTIVE This study aimed to investigate the effectiveness and tolerability of the combination of electroconvulsive therapy (ECT) in patients with clozapine-treated schizophrenia. METHODS Patients with clozapine-treated schizophrenia during five years of pre-determined period were recruited from Electronic Medical Record. Clinical effects of acute ECT on psychotic symptoms were investigated. We also tried to identify predictive variables requiring maintenance treatment of ECT. RESULTS Fourteen patients received ECT and clozapine and sixteen were treated with clozapine alone. In the ECT group, which could be refined as clozapine-resistance, PANSS total score was significantly reduced by 19.0±9.9 points, corresponding to a reduction rate of 18.5±8.3%. The clinical remission defined as 20% PANSS reduction criteria was achieved at 42.9%. The subscale factors were significantly reduced, among which the negative symptom was the least. There was no difference in demographic and clinical information between patients receiving and not receiving maintenance ECT, and not all patients seemed to need maintenance ECT if clozapine is continued. CONCLUSIONS Combination of ECT and clozapine in patients with clozapine-resistant schizophrenia resulted in a rapid and substantial reduction of psychotic symptoms. Further studies are needed to improve the effectiveness and tolerability of ECT.