Background -Ventricular tachyarrhythmias long enough to cause implantable cardioverter defibrillator(ICD) shocks are generally thought to progress to cardiac arrest. In previous ICD trials, shocks have been considered an appropriate surrogate for sudden cardiac death(SCD) because the number of shocks has been thought to be equivalent to the mortality excess in patients without ICDs. The practice of equating ICD shocks with mortality is controversial and has not been validated critically. Methods and Results -The Defibrillators in Non-Ischemic Cardiomyopathy Treatment Evaluation(DEFINITE) trial was a prospective, randomized, multicenter trial of ICD therapy in 458 patients with nonischemic cardiomyopathy. Patients were randomized to receive standard medical therapy(STD) or STD plus an ICD. Shock electrograms were reviewed, and the cause of death was evaluated by a separate blinded events committee. There were 15 SCD or cardiac arrests in the STD group and only 3 in the ICD arm. In contrast, of the 229 patients randomized to an ICD, 33 received 70 appropriate ICD shocks. Patients in the ICD arm were more likely to have an arrhythmic event(ICD shock plus SCD) than patients in the STD arm(hazard ratio 2.12, 95%CI 1.153 to 3.893, P=0.013). The number of arrhythmic events when one includes syncope as a potential arrhythmic event was similar in both groups(hazard ratio 1.20, 95%CI 0.774 to 1.865, P=0.414). Approximately the same number of total events was noted in each arm when we compared syncope plus SCD/cardiac arrest in the STD arm with SCD plus ICD shocks plus syncope in the ICD arm. Conclusions -Appropriate ICD shocks occur more frequently than SCD in patients with nonischemic cardiomyopathy. This suggests that episodes of nonsustained ventricular tachycardia frequently terminate spontaneously in such patients. Background -Ventricular tachyarrhythmias long enough to cause implantable cardioverter defibrillator (ICD) shocks are generally thought to progress to cardiac arrest. In previous ICD trials, shocks have been considered an appropriate surrogate for sudden cardiac death (SCD) because the number of shocks has been The practice of equating ICD shocks with mortality is controversial and has not been validated critically. Methods and Results-The Defibrillators in Non-Ischemic Cardiomyopathy Treatment Evaluation (DEFINITE) trial was a prospective , randomized, multicenter trial of ICD therapy in 458 patients with nonischemic cardiomyopathy. Patients were randomized to receive standard medical therapy (STD) or STD plus an ICD. Shock electrograms were reviewed, and the cause of death was evaluated as a separate blinded events committee . There were 15 SCD or cardiac arrests in the STD group and only 3 in the ICD arm. In contr ast, of the 229 patients randomized to an ICD, 33 received 70 appropriate ICD shocks. Patients in the ICD arm were more likely to have an arrhythmic event (ICD shock plus SCD) than patients in the STD arm (hazard ratio 2.12, 95% CI 1.153 to 3.893, P = 0.013). The number of arrhythmic events when one includes syncope as a potential arrhythmic event was similar in both groups (hazard ratio 1.20, 95% CI 0.774 to 1.865, P = 0.414). Approximately the same number of total events was noted in each arm when we syncope plus SCD / cardiac arrest in the STD arm with SCD plus ICD shocks plus syncope in the ICD arm. Conclusions-Appropriate ICD shocks occur more frequently than SCD in patients with nonischemic cardiomyopathy. suggests that episodes of nonsustained ventricular tachycardia frequently terminate spontaneously in such patients.