BACKGROUND: Patients with severely depressed left ventricular ejection fractions (LVEFs) receive implantable cardioverter-defibrillators (ICDs) for the primary prevention of sudden death. In some patients, however, LVEF may improve or even normalize over time. Limited data are available on the incidence of appropriate antitachycardia therapy, including pacing and shock, in these patients. METHODS AND RESULTS: We retrospectively identified consecutive patients at our institution with an ICD for primary prevention who had LVEF measurement available at initial implantation and at the time of generator replacement. None of these patients had ever received appropriate antitachycardia therapy before generator replacement. The incidence of appropriate antitachycardia therapy after generator replacement was assessed. Of the 125 patients who received generator replacement, 53 (42%) received an ICD and 72 (58%) a cardiac resynchronization therapy-defibrillator (CRT-D). Among them, 30 (21%) had LVEF normalized to >/=50%, 25 (17%) had LVEF partially improved to 36%-49%, and 70 (63%) had LVEF that remained depressed at