Background—This historical cohort study evaluated the benefit of bilateral internal mammary artery (BIMA) grafts in coronary bypass grafting (CABG) for patients with diabetes.Methods and Results—We performed elective, isolated, primary, multiple CABG using skeletonized internal mammary artery (IMA) grafts for multivessel disease in 1131 patients, 467 (41.3%) of whom had type 2 diabetes mellitus. The early and long-term results were compared between 277 patients with diabetes using single IMA (SIMA) grafts and 190 using BIMA grafts (median follow-up, 8.1 years). Hospital mortality was similar in both groups. Early patency rate of all grafts was significantly higher using BIMA than using SIMA (97.7% versus 93.8%,P=0.0012). Survival rates were not significantly different between SIMA and BIMA groups. Late cardiac mortality was significantly higher in patients with low ejection fraction (0.4 or lower) compared with preserved ejection fraction (higher than 0.4) (P=0.0001). In patients with preserved ejection fraction, 10-year survival rate was significantly higher using BIMA than using SIMA (87.8±3.5% versus 75.2±3.4%,P=0.04), and 10-year all death–free or repeat CABG or recurrent myocardial infarction–free rate was significantly higher using BIMA than using SIMA (86.6±3.6% versus 69.0±3.7%,P=0.0086). The hazard ratio for all death or repeated CABG or recurrent myocardial infarction in patients with preserved ejection fraction was markedly lower in the BIMA group (0.53; 95% CI, 0.31 to 0.9;P=0.019).Conclusions—Skeletonized BIMA grafts are beneficial in coronary revascularization for diabetic patients with preserved ejection fraction but have limited survival benefit for those with reduced ejection fraction attributable to high cardiac mortality.