A significant proportion of implantable cardioverter defibrillator (ICD) recipients ultimately receive device therapies. Determinants and mechanisms of syncopal events in these patients are not established. To address this issue, we reviewed prospectively collected data on 114 ICD recipients who received device therapies. There were 99 men and 15 women with a mean age of 63+/-10 years. Ejection fraction was 29+/-13%. At initial presentation, 74 patients (65%) had syncope during arrhythmia. Of 114 patients, 18 patients (16%) subsequently experienced syncope during device therapies over a period of 35+/-24 months of follow-up. Arrhythmias during syncopal events documented by telemetry, event recorders, RR intervals, or ICD stored electrograms were obtainable in 13 patients, and included sustained monomorphic ventricular tachycardia (VT) in 10 patients, ventricular fibrillation (VF) in 2 patients, and atrial fibrillation in 1 patient. The explanations for arrhythmia-related syncope included ICD proarrhythmia in 8 patients (62%), rapid monomorphic VT in 3 (23%), and VF in 2 patients (15%). Patients with syncope could not be differentiated from those without syncope by demographic, clinical, or electrophysiologic variables. There was no significant difference in mortality between patients with or without syncope (39% vs 25%, p = 0.25). Syncope is not uncommon in ICD recipients who receive device therapies. When it occurs, syncope is often due to ICD proarrhythmia; hence, it is potentially preventable by meticulous device programming.