OBJECTIVES: Diabetes mellitus is a risk factor for coronary artery disease, but its role following coronary artery bypass grafting (CABG) is still unclear and few data on long-term outcomes are available. This study aimed to evaluate the impact of diabetes on long-term outcomes after CABG. METHODS: The PRIORITY (PRedictIng long-term Outcomes afteR Isolated coronary arTery bypass surgerY) project is an observational cohort study merging 2 prospective multicentre studies on isolated CABG. Follow-up information was obtained through administrative databases and was truncated 10 years after the intervention. Baseline differences between patients with and without diabetes were balanced with inverse probability of treatment weighting. RESULTS: The cohort consisted of 10 989 patients with complete follow-up information who underwent isolated CABG (diabetes 32.3%). Diabetes did not affect short-term mortality [odds ratio (OR) 0.90, 95% confidence interval (CI) 0.73-1.10] and repeat revascularization (OR 0.79, 95% CI 0.42-1.49), while it is related to lower incidence of 30-day major adverse cardiac and cerebrovascular events (OR 0.67, 95% CI 0.60-0.76), acute myocardial infarction (OR 0.60, 95% CI 0.51-0.70) and stroke (OR 0.47, 95% CI 0.28-0.77). Diabetic patients had a higher long-term risk for major adverse cardiac and cerebrovascular event [weighted hazard ratio (HR) 1.31, 95% CI 1.26-1.37], mortality (HR 1.45, 95% CI 1.37-1.53), as well as stroke (HR 1.38, 95% CI 1.25-1.53) and myocardial infarction (HR 1.39, 95% CI 1.26-1. 53). Diabetes had not been associated with an increased incidence of repeated revascularization up to 10 years (HR 1.04, 95% CI 0.96-1.12). CONCLUSIONS: Diabetic patients had worse long-term outcomes. Diabetes may have a greater negative impact on micro-vasculopathy than grafts, as evidenced by the increased long-term incidence of myocardial infarction without affecting myocardial revascularization.