In order to determine the incidence of complications of implantable cardioverter defibrillator (ICD) therapy, 241 patients with a total of 353 ICD implantations were followed for 24 +/- 20 months. Complications were defined as any untoward effects experienced by the patient related to the ICD implantation and function or death within 4 weeks of implant or before hospital discharge. During follow-up, 129 of 241 patients (53%) had a total of 166 complications. An operative procedure was required to correct the complication in 50 of the 241 patients (21%). No patient died intraoperatively, but eight patients died within 4 weeks postoperatively or before hospital discharge. ICD infection required removal of the device in 13 patients (5%). Twenty-six patients (11%) had postoperative respiratory complications. Postoperative bleeding and/or thrombosis occurred in 11 patients (4%). Endocardial lead migration was observed in nine patients (4%) and lead adapter or insulation break occurred in nine patients (4%). Fifty-four patients (22%) experienced ECG-documented discharges for non-VT rhythm. In conclusion, although perioperative mortality in ICD patients is low, the majority of patients have complications during follow-up. Recognition of these complications may allow for advances in ICD technology and management strategies to avoid their occurrence.