OBJECTIVES The aim of this study was to understand the causes of implantable cardioverter defibrillator (ICD) failure and complications so that adverse events, including unnecessary death, can be prevented. BACKGROUND Sudden death may occur if an ICD fails to treat life-threatening ventricular arrhythmias. METHODS The United States Food and Drug Administration Manufacturer and User Facility Device Experience Database was searched for ICD devices and the search term "death." The search yielded 212 death events involving 100 ICD pulse generator and lead models from five manufacturers. These death events were associated with (A) ICD devices for which pulse generator interrogation data and/or the results of the manufacturers analysis of returned devices were available; (B) ICD devices for which neither interrogation data nor the results of the manufacturer's analysis were reported; and (C) normally functioning ICDs that had been deactivated. RESULTS (A) A total of 103 (69%) of 150 death events were associated with defective pulse generators or high-voltage leads. Most (34/42 [81%]) apparently sudden or arrhythmic death events were associated with high-voltage lead failure; other deaths were related to pulse generator failure (8/42 [19%]) caused by electronic component defects. (B) A total of 21 of 51 death events were related to a manufacturer's recall; all deaths were arrhythmic but without allegation of device failure. (C) Eleven death events occurred in patients whose pulse generators were found to be off or deactivated; these devices appeared to have been deactivated accidentally or by exposure to magnetic fields, or they were not reactivated after elective surgery. CONCLUSIONS ICD device failure and unintended pulse generator deactivation have resulted in unnecessary deaths. Although these deaths may be infrequent, improved devices and follow-up techniques are needed. The magnet deactivation feature probably is unsafe, and health professionals and patients should be cautioned.