Background Bilateral internal thoracic artery (BITA) grafting in patients with diabetes mellitus is controversial because of a higher risk for sternal infection. The purpose of this study is to compare the outcome of BITA grafting to that of single ITA (SITA) grafting in patients with diabetes. Methods Between 1996 and 2010, 964 diabetic patients with multivessel disease who underwent primary coronary artery bypass graft surgery with BITA were compared with 564 patients who underwent coronary artery bypass graft surgery with SITA and saphenous vein grafts. Results The SITA patients were older, more often female, more likely to have chronic obstructive pulmonary disease, ejection fraction 30% or less, insulin-dependent diabetes, recent myocardial infarction, renal insufficiency, peripheral vascular disease, and emergency operation. The BITA patients more often underwent coronary artery bypass graft surgery with three or more grafts. The two groups had similar operative mortality, 2.6% BITA versus 3.0% SITA, and sternal infection, 3.1% versus 3.9%, respectively. The mean follow-up was 12.2 ± 4.3 years. Unadjusted Kaplan-Meier 10-year survival of the BITA group was better than that of the SITA group (65.3% ± 3.1% versus 55.5% ± 4.5%, respectively; p = 0.004), After propensity score matching (490 well-matched pairs), Kaplan-Meier 10-year survival was not significantly different between the matched groups; however, the Cox-adjusted survival of the BITA patients was better (hazard ratio 0.729, 95% confidence interval: 0.551 to 0.964, p = 0.027). Conclusions The findings of this large cohort study suggest that the long-term outcome of patients with diabetes and multivessel disease who undergo BITA grafting is better than that of diabetic patients who undergo coronary artery bypass graft surgery with SITA and saphenous vein grafts.