Left ventricular ejection fraction (LVEF) is accepted as an important prognostic marker in patients (pts) with implantable cardioverter-defibrillators (ICD). The impact of this therapeutic approach in the survival of pts with life-threatening arrhythmias and severe left ventricular dysfunction remains a matter of discussion. OBJECTIVE To evaluate the long-term clinical implications of severe left ventricular dysfunction in pts with an ICD implanted for secondary prevention of sudden cardiac death (SD). METHODS Out of 70 pts undergoing ICD implantation in our institution over four consecutive years, we studied 24 pts with LVEF <35% and a post-ICD follow-up of >12 months (87.5% male; age 62.79 years). The index arrhythmia was ventricular tachycardia in 19 cases and SD with ventricular fibrillation in 5 cases. The underlying disease was ischemic cardiomyopathy (n=19), dilated cardiomyopathy (n=4) and hypertensive heart disease (n=1). Mean LVEF at the time of implant was 25 +/- 7% (between 11% and 34%; NYHA class II/III in 83.3%). A du chamber system was implanted in 5 cases, and an ICD plus cardiac resynchronization pacing in 2 cases. There was no perioperative mortality. At the time of discharge, 71.2% of the pts were taking amiodarone and 66.7 % beta-blockers. During a 38 +/- 16-month follow-up (4 appointments/year), we analyzed the following parameters: rehospitalization for cardiovascular cause, appropriate ICD shocks, inappropriate detections/therapy, nonfatal major arrhythmic events (arrhythmic storm, therapeutic exhaustion, recurrent ventricular tachycardia), cardiac mortality, SD and total mortality. RESULTS Forty-five readmissions (1.9 +/- 2.3/pt) occurred in 14 pts (58%), 24.4% due to congestive heart failure. Appropriate ICD shocks (without hospitalization) occurred in 62.5% of the pts, 16.6% had inappropriate therapy (50% because of increased heart rate due to atrial fibrillation) and 37.5% suffered nonfatal major arrhythmic events. Death due to SD was 4.2%, cardiac mortality 12.5% and total mortality 25%. CONCLUSIONS Severe left ventricular dysfunction is common in ICD pts. During long-term follow-up, the majority of these pts receive appropriate ICD shocks, which emphasizes the importance of SD prevention in this population. The frequent documentation of supraventricular arrhythmias (causing inappropriate ICD therapy) and nonfatal major arrhythmic events also reflects the presence of a worse arrhythmic substrate in this subgroup. Despite the poor initial prognosis associated with ventricular tachyarrhythmias in pts with severe left ventricular dysfunction, ICD therapy may contribute to a better long-term clinical outcome.