OBJECTIVE: To determine whether patients with life threatening ventricular tachyarrhythmias, impaired left ventricular function, and severe heart failure will benefit from implantable cardioverter-defibrillator (ICD) treatment. DESIGN: 410 patients were followed up after ICD implant. Left ventricular function was assessed by the New York Heart Association (NYHA) functional class of heart failure: 50 patients (12%) were in NYHA I-II, 151 (37%) in NYHA II, 117 (29%) in NYHA II-III, and 92 (22%) in NYHA III. Epicardial ICD implantation was performed in 209 patients (51%) and 201 patients (49%) received non-thoracotomy ICDs. RESULTS: Perioperatively, 12 patients (3%) died, more often after epicardial ICD implant (11/209 patients, 5%) than after transvenous implant (1/201 patients, < 1%) (P < 0.05). During a mean (SD) follow up of 28 (24) months (range < 1 to 114 months), 90 patients (23%) died: nine (2%) died from sudden arrhythmia; five (1%) also died suddenly but probably not from arrhythmic causes; 55 (14%) died from cardiac causes (congestive heart failure, myocardial reinfarction); 21 (5%) died from non-cardiac causes. The three year, five year, and seven year survival was 92-96% for arrhythmic mortality in NYHA class I, II and III, compared to a three year survival of 94% and a five year and seven year survival of 84% for patients in NYHA class II-III. 338 patients (82%) received ICD shocks (21 (SD 43) shocks per patient); patients in NYHA class II (83%), class II-III (84%), and class III (90%) received ICD discharges more often than those in class I-II (64%) (P < 0.05). The mean (SD) time interval between ICD implant and the first ICD shock was shorter in NYHA class II (16 (17) months), class II-III (19 (27) months), and class III (16 (19) months) than in class 0-I (22 (24) months) (P < 0.05). CONCLUSIONS: Patients with mild, moderate, and severe left ventricular dysfunction benefit from ICD treatment and these patients survive for a considerable time after the first shock. Survival is influenced by the degree of left ventricular dysfunction; aggressive treatment of heart failure is necessary as well as ICD therapy.