Our aim was to investigate the outcome of patients with diabetes undergoing coronary artery bypass grafting (CABG) surgery in a whole population with main focus on long-term mortality and complications. This was a nationwide retrospective analysis of all patients who underwent isolated primary CABG in Iceland between 2001 and 2016. Overall survival together with the composite end point of major adverse cardiac and cerebrovascular events was compared between patients with diabetes and patients without diabetes during a median follow-up of 8.5 years. Multivariable regression analyses were used to evaluate the impact of diabetes on both short- and long-term outcomes. Of a total of 2060 patients, 356 (17%) patients had diabetes. Patients with diabetes had a higher body mass index (29.9 vs 27.9 kg/m2) and more often had hypertension (83% vs 62%) and chronic kidney disease (estimated glomerular filtration rate ≤60 ml/min/1.73 m2, 21% vs 14%). Patients with diabetes had an increased risk of operative mortality odds ratio 2.52, 95% confidence interval (CI) 1.27-4.80 when adjusted for confounders. 5-Year overall survival (85% vs 91%, P < 0.001) and 5-year freedom from major adverse cardiac and cerebrovascular events were also inferior for patients with diabetes (77% vs 82%, P < 0.001). Cox regression analysis adjusting for potential confounders showed that the diagnosis of diabetes significantly predicted all-cause mortality hazard ratio (HR) 1.87, 95% CI 1.53-2.29 and increased risk of major adverse cardiac and cerebrovascular events (HR 1.47, 95% CI 1.23-1.75). Patients with diabetes have significantly lower survival after CABG, both within 30 days and during long-term follow-up.