Abstract Introduction: The most commonly used biomarker for long-term glycemic control and for the diagnosis of diabetes mellitus is glycosylated hemoglobin. Determining its level may provide important information on the risk of complications related to this disease. Research shows that glycosylated hemoglobin may also be prognostic in terms of postoperative outcomes in cardiac surgery patients. Objective: The aim of this study is to assess the effect of an increased level of glycosylated hemoglobin on physical performance in patients after cardiac surgery. Materials and methods: 109 patients participated in the study, divided into two groups: study - 27 people and control - 82 people. The decisive factor in belonging to the appropriate group was the level of glycated hemoglobin. Results: Data analysis showed that there were statistically significant differences between the study group and the control group in the mean result of the 6-minute walk test before admission to the ward (p = 0.005), in the results of systolic blood pressure measurement before the test (p = 0.011) and after performing this test (p = 0.012), in the diastolic blood pressure results before (p <0.001) and after the test (p = 0.001). The differences turned out to be statistically significant also in the heart rate measurement after the 6-minute walk test on admission (p = 0.019). Conclusion: Elevated levels of glycosylated hemoglobin may be a factor affecting exercise tolerance in patients after cardiac surgery, which results in reduced effectiveness of cardiac rehabilitation. Keywords: rehabilitation, cardiology, walk test