Heart failure (HF) is a common condition in elderly patients. Despite great improvements in medical therapy, HF mortality remains high. Implantable cardioverter defibrillator (ICD) significantly lengthens the survival rate of subjects with severe HF, but little evidence exists on its effect in elderly persons. Aim of this study was to compare the age-related determinants of prognosis in a large population of patients with ICD. We divided all patients who underwent an ICD implantation in 117 Italian centers of the “ClinicalService Project” into 3 age groups (<65, 65 to 74, ≥75 years), and collected clinical and instrumental variables at baseline and during follow-up (median length: 27 months). Between 2004 and 2011, 6,311 patients were enrolled (5,174 men; left ventricular ejection fraction 29% ± 9%); 1,510 subjects were ≥75 years (23.9%; mean age 78 ± 3 years). The prevalence of co-morbidities increased with age. HF was most frequently due to coronary artery disease in the elderly, who also showed the worst New York Heart Association class. At multivariate analysis, older age, coronary artery disease, chronic obstructive pulmonary disease, chronic renal failure, diabetes, complex ventricular arrhythmias, and left ventricular ejection fraction were significant predictors of all-cause mortality. After adjustment, the hazard ratioage group for mortality was 22.6% less than at univariate analysis. When groups were analyzed separately, age alone predicted mortality in the oldest. In conclusion, a large proportion of our population was aged ≥75 years. Mortality was related to age and several co-morbidities, except for the oldest patients in whom age alone resulted predictive.