Implantable cardioverter-defibrillator (ICD) therapy for primary prevention of sudden cardiac death is not routinely recommended within 90 days of coronary artery bypass grafting (CABG) because of the possibility of an improvement in left ventricular ejection fraction (EF) to>35% after revascularization. We sought to determine the incidence and predictors of EF improvement to >35% after isolated CABG in patients who had a preoperative EF ≤35%. We studied 375 patients who underwent CABG at a tertiary institution and had an echocardiogram preoperatively and postoperatively. Of these, 74 patients (20%) with a preoperative EF ≤35% were included in this analysis. Improvement in EF was defined as postoperative EF >35%. In the overall study population (n = 74), mean EF improved from 28 ± 6% preoperatively to 36 ± 12% postoperatively (p <0.0001). A total of 38 patients (51%) had postoperative improvement in EF to >35% (mean EF in these patients increased from 30 ± 5% to 46 ± 8%; p <0.0001). Patients with EF improvement had a higher preoperative EF than those with no improvement (30 ± 5% vs 26 ± 7%, p <0.005). Improvement in EF was 5 times more likely in patients with preoperative EF 26% to 35% (odds ratio 4.95, 95% CI 1.73 to 14.1; p = 0.003) than those with preoperative EF ≤25%. Other clinical characteristics were not significantly different between patients with versus without EF improvement. In conclusion, more than half of the ICD-eligible patients who underwent CABG improved their EF to >35% after surgery and became ineligible for a primary prevention ICD. EF improvement was unlikely in patients with preoperative EF <25%.