Objective: In this study the antioxidant enzyme [catalase (CAT), superoxide dismutase (SOD), and glutathione peroxidase (GPx) ] levels at rest in patients with syndrome X and coronary slow flow are measured. Then it has been investigated whether there is any enzymatic difference between the normal controls and syndrome X patients or patients with coronary slow flow and ascertain if exercise has any effects on the antioxidant enzyme levels. Methods: Fifty-five patients were included in this prospective observational controlled study. Patients were divided into 3 groups: Group 1- normal controls (n=20); Group 2-patients with coronary slow flow (n=20); and Group 3-patients diagnosed with syndrome X (n=15). In all patients, blood samples were collected at rest and after maximal exercise. The antioxidant enzymes (SOD, CAT, Gpx) in the erythrocytes were studied for these three groups of blood sample. Statistical analysis was performed using Student t-test, Mann-Whitney U and Chi-square tests, Kruskal- Wallis variance analysis and ANOVA. Results: Under basal conditions the lowest SOD and GPx levels were measured in the 2nd Group, whereas significant differences in paired comparisons were observed only between the 2nd and 3rd Groups (p=0.024 vs p<0.01, respectively) during paired comparisons. The post-exercise SOD levels were decreased significantly in the 3rd Groups when compared with the basal concentrations (p=0.014), however no significant pre- and post-exercise differences were observed in the CAT and GPx concentrations (p>0.05). Conclusion: The post-exercise SOD level when compared with basal SOD levels were decreased significantly in the syndrome X group, however no differences were observed between the other groups. This can be interpreted as the reduction in the exercise related symptoms and ischemic findings are resulting from the decrease of SOD activity.