Objectives. This report describes the sensing/pacing lead complications that developed during a worldwide clinical trial of a new impiantable cardioverter-defibrillator. Background. The reliability of the leads used for sensing and pacing with the implantable cardioverter-defibrillator has not been adequately studied. Methods. The Guardian ATP 4210 was implanted in 302 patients. The sensing/pacing leads consisted of either two unipolar epicardial electrodes or a bipolar endocardial electrode from a variety of manufacturers. Results. During a mean follow-up period of 380 days, 39 patients (12.9%) required reoperation because their device developed sensing/pacing lead system complications. The most common clinical presentation was device oversensing (multiple tachycardia or noise detections or inappropriate shocks), which was observed in 27 patients, whereas elevated pacing thresholds were seen in 10 patients. Forty-one (11.8%) of 347 implanted lead systems required revision. The mean time to revision was 156 ± 145 days. Actuarial lead survival rate at 1 and 3 years was 89% and 79%, respectively. Epicardial lead systems required significantly (p < 0.05) more revision than did endocardial systems, but when adapter problems were excluded, the revision rates of epicardial and endocardial were similar. Causes of lead system failures included adapter connection problems, lead dislodgement and insulation disruption. Predictors of lead revision were use of an epicardial lead system or an adapter. Conclusions. A high rate of sensing/pacing lead complications was found with this newer generation implantable cardioverterdefibrillator. The enhanced diagnostic and data storage capabilities of this implantable cardioverter-defibrillator facilitated the recognition and troubleshooting of these complications. These findings emphasize the need for careful surveillance and testing of implantable cardioverter-defibrillator sensing/pacing leads during follow-up.