Background: Endothelial dysfunction is an independent predictor of future cardiac events. Material/Methods: We evaluated the relationship between flow-mediated dilation (FMD) in brachial artery and coronary risk factors in 93 patients (70 males, mean age: 62 +/- 8 years) with ACS treated with primary angioplasty (PCI). The patients were divided into 2 subgroups: 43 patients with diabetes mellitus type 2 (DM) and 50 non-diabetics (non-DM). Patients were examined on the 3(rd) day after ACS and after 6 months. FMD on the 3(rd) day were significantly lower in DM than in non-DM (5.8 +/- 2.2% vs. 8.8 +/- 4.9%, p= 0.0007) and after 6 months (6.2 +/- 2.6% vs. 9.4 +/- 4.4%, p < 0.0001). It was also observed that the improvement of FMD in both groups after a 6-month follow-up inversely correlated with the increase of left ventricular end-diastolic volume (LVEDV) (r = -0.41, p < 0.001). Results: There was an inverse relationship between FMD and age (r = -0.26, p < 0.01), BMI (r = -0.26, p < 0,005), total cholesterol (r = -0.56, p < 0.001) and LDL cholesterol (r = -0.53, p < 0.001). There was no relationship between triglycerides, hypertension and history of smoking. In the DM group, FMD negatively correlated with HbA1c (r = -0.68, p < 0.001). Restenosis rate was significantly higher in the DM group (19% vs. 6%, p < 0.001) but there was no relationship between FMD and restenosis. Conclusions: Impaired FMD is more significant in diabetics than in non-diabetic patients with ACS. Lack of improvement of FMD after acute coronary syndrome can be a predictor of detrimental left ventricular remodeling in patients with ACS.