Background and aim of the study Approximately 30% of patients undergoing cardiac surgery have a history of diabetes and 60-80% of patients without diabetes have stress hyperglycemia. We examined patients undergoing cardiac surgery to determine the presence of stress hyperglycemia and its relationship to tissue perfusion. Methods Hemodynamic parameters, central venous oxygen saturation, lactate,oxygen delivery and consumption, oxygen extraction rate were analyzed at four intraoperative time points. Results The stress-induced hyperglycemic response during cardiac surgery was more severe in patients without diabetes. When focusing on the oxygen extraction rate in terms of tissue oxygenation, diabetic patients had 1.22 times higher and significant oxygen extraction rate than non-diabetic patients. Conclusions Although lactate values were slightly higher and central venous oxygen saturation were slightly lower in the diabetic group, considering the fact that oxygen extraction rate reflects the total outcome of small changes in all these parameters, we can emphasize the conclusion that diabetic patients undergoing cardiac surgery have greater tissue oxygen demand/supply imbalance compared to non-diabetic patients. In our study, this tissue oxygenation defect in diabetic patients was not found to be directly correlated with blood glucose levels. Perhaps, even if the disease is under control, the negative effects of diabetes on all systems have accumulated and led to such a result.