Introduction: Open heart surgery in patients with diabetes mellitus (DM) is associated with a higher mortality and morbidity than other patients. Diabetes mellitus (DM) is present in 30 to 40% of patients undergoing coronary bypass surgery (CABG). In this study, we aimed to clarify the relationship between preoperative glycohemoglobin (HbA1C) levels and postoperative acute renal failure (ARF) in patients with DM undergoing isolated coronary bypass surgery. Methods: We retrospectively enrolled a total of 295 patients who underwent elective, isolated CABG between January 2014 and February 2017 in our clinic and whose information was recorded. DM was detected in 118 of 295 patients. These patients were divided into two groups as Group 1 (HbA1c levels <7%, n = 72) and Group 2 (HbA1c levels >7%, n = 46). All patients were treated with standard insulin therapy after consulting the internal medicine department before the operation. Results: Of the 118 patients included in the study, 82 were males and 36 were females. There were 72 patients (51 M, 21 F) in Group 1 and 46 patients (31 M, 15 F) in Group 2. The mean age was 62.4 +/- 3.2 years in Group 1 and 61.5 +/- 4.5 years in Group 2. The mean duration of DM diagnosis was 10.2 +/- 3.3 years in Group 1 and 11.7 +/- 2.6 years in Group 2. The mean duration of hospitalization in intensive care unit was 6.10 +/- 2.3 days in Group 1 and 9.1 +/- 2.5 days in Group 2, which was found to be statistically significant (p=0.008). Discussion and conclusion: Hemodialysis (HD) may be required after coronary bypass surgery in diabetic patients. Although there is no direct correlation between high HbA1c levels and postoperative HD, we believe that these patients should be more closely monitored with more frequent measurements of urea, creatinine, blood gas and electrolyte levels.