Background: Patients with severely depressed left ventricular ejection fractions (LVEF) receive implantable cardiac defibrillators (ICD) for the primary prevention of sudden death. Limited data are available on 1) the incidence of late recovery of LVEF in these patients and 2) the incidence of appropriate anti-tachycardia therapy including pacing and shock in patients with normalized LVEF. Methods: We retrospectively identified 154 consecutive patients with an ICD for primary prevention who had LVEF available at initial implantation and prior to generator replacement. The incidence of appropriate anti-tachycardia therapy after generator replacement was assessed. Results: Of the 154 patients (65 ± 14 years, females 25%), 19 (12%) had improvement in their LVEF from 26 ± 9% to = 55%. None of these individuals experienced any appropriate anti-tachycardia therapy during a follow-up period of 28 ± 18 months. Among the remaining 135 patients, with depressed LVEF (25 ± 7%), 30 individuals (22%) had at least one appropriate anti-tachycardia therapy during a follow-up period of 25 ± 18 months. The difference in appropriate anti-tachycardia therapy between the two groups was highly significant (p = 0.02). Compared to patients whose LVEF remained depressed, patients with normalized LVEF trended towards being female (37% vs. 24%, p =0.26), with a lower prevalence of ischemic cardiomyopathy (53% vs. 73%, p = 0.1). Conclusion: 12% of the patients with depressed LVEF, who received ICD initially for primary prevention of sudden death, had normalized LVEF at the time of generator replacement. None of these patients received appropriate anti-tachycardia therapy during a follow-up period of 28 ± 18 months. The practice of routine replacement of generator in these patients may need to be re-assessed on an individual basis. Longer follow-up in a larger population is needed to confirm these findings.