Experimental studies have shown that smoking was related to endothelial dysfunction via oxidative stress. However, the degree of oxidative stress to be associated with endothelial dysfunction is unknown. Oxidative stress index (OSI) might be a useful and easy way of determining the endothelial dysfunction. Hence, we aimed to evaluate the relationship between OSI and flow mediated dilatation (FMD) in smoking healthy male volunteers. Eighty smoking healthy male volunteers were enrolled in the study. Participants were classified as having normal and abnormal FMD response. In an univariate analysis; systolic and diastolic blood pressures, C-reactive protein (CRP), low-density lipoprotein cholesterol, OSI and lipid peroxidation (LPO) levels were predictive for abnormal FMD response. In a multivariable logistic regression analysis with forward stepwise method, OSI (OR: 3.194, 95% CI: 1.710-5.966, p<0.001) and CRP (OR: 2.082, 95% Cl: 1.101-3.939, p 0.024) were found to be independent parameters for predicting abnormal FMD response in young male smokers. The optimal cut-off value of OSI for detecting abnormal FMD response was found to be >3.35, with 100 % sensitivity and 84.1 % specificity. We have shown that critical endothelial dysfunction can easily be detected by OSI in individuals, at risk for developing coronary artery disease, such as smokers (Tab. 3, Fig. 3, Ref. 30). Text in PDF www.elis.sk.