Electroconvulsive therapy (ECT) is an effective, safe, and controversial treatment of severe depression. In order to further evaluate its safety, the effect of ECT on serial electrocardiograms and serum cardiac enzyme values was studied prospectively in 29 patients. Neither persistent electrocardiographic changes nor elevations in creatine phosphokinase or serum glutamic oxalaminase transaminase levels were observed following 85 treatments. Twenty-four percent of our patients had stable, preexisting cardiovascular disease, which included conduction system disease, recent myocardial infarction, and depressed ventricular function. Electroconvulsive therapy was well tolerated by all of these patients. The proposed mechanisms for transient or persistent electrocardiographic changes and cardiovascular complications of ECT are reviewed.