Objective: The study aim was to investigate the effect of blood glucose level on atherosclerotic lesion distribution and the contribution to the operative mortality/morbidity in diabetic patients who underwent coronary artery bypass grafting (CABG). Materials and Methods: Between 1986-2003, a total of 2095 patients with diabetes mellitus underwent CABG. The analysis was carried out retrospectively from the clinical records. The patients were divided into four groups according to the blood glucose levels that were obtained when they first applied to hospital; Group 1 (492 patients with blood glucose < 120 mg/dl), group 2 (1112 patients with blood glucose - 120-200 mg/dl, group 3 (261 patients with blood glucose 201-250 mg/dl) and group 4 (230 patients with blood glucose > 250 mg/dl). “One way ANOVA” test was used for the statistical analysis of continuous variables and Chi-square test was used for analyzing the categorical variables. Results: Emergent operation rate due to acute ischemia gradually increased from Group 1 to Group 4 and reached 6.6% in Group 4 (p=0.005). Operation time and the duration of cardiopulmonary bypass and cross clamp were significantly longer in patients with high blood glucose levels (p<0.05). Necessity for inotropic drug administration postoperatively (p<0.05) and mechanical support (p<0.05) were significantly higher also. The hospital mortality in group 3 was 9.6% and in group 4 was 11.3% (p=0.09). No statistically significant difference was found in terms of morbidity between the groups (p>0.05). The multi-vessel coronary artery disease was more common in groups with high blood glucose level (p<0.05). As the blood glucose level raised, patients were more frequently (p<0.05) confronted with distal left anterior descending artery, middle circumflex artery and right coronary artery lesions. Conclusion: Uncontrolled blood glucose level not only increased the perioperative complications but also the incidence of middle and distal coronary artery lesions. It is necessary to diagnose and aggressively treat the high blood glucose level especially before the CABG.