Objectives: Hyperglycemia is a prominent feature among patients exposed to major stress such as in cardiac surgery. The implementation of a continuous glucose monitoring system (CGMS) for glucose monitoring during cardiac surgery was assessed. Subjects and Methods: Fifty-nine consecutive patients who underwent cardiac surgery were monitored by CGMS. Patients' fluid glucose content, drug requirements, and hemodynamic and physiologic parameters were evaluated. Results: Of the 59 patients, 32 completed the monitoring with CGMS. Patients were divided into three groups: diabetes patients, patients without diabetes history who developed significant hyperglycemia perioperatively, and patients who did not develop hyperglycemia. Hyperglycemia was most frequently observed postoperatively. Hyperglycemic patients required significantly more insulin (81±40 vs. 34±37 units, P=0.005) and experienced an increased early complication rate, although this difference was not significant. CGMS erroneously detected late-phase operative and immediate postoperative hypoglycemia in approximately one-third of patients as reflected from venous blood sample measurements. Conclusions: CGMS enables close monitoring and optimal control of blood glucose among patients undergoing major cardiac surgery, although its reliability is limited during the cardiac surgery phase and in the early postoperative period, because of incorrect hypoglycemic readings.