Background Peripheral endothelial dysfunction (ED) quantified by the determination of flow-mediated dilation (FMD%) of the brachial artery with the use of high-resolution ultrasound is an early marker of atherosclerosis. Although a positive correlation with coronary artery disease (CAD) has been reported, the unanswered clinical question is the validity of FMD% as a screening test in patients with clinical suspicion of CAD. Thus the aim of this study was to determine the predictive value of FMD% compared with angina pectoris, exercise electrocardiography, and myocardial perfusion imaging. Methods and Results In this pilot study, we measured ED in 122 patients scheduled for coronary angiography by using high-resolution ultrasound (13 MHz). We defined ED as FMD% ≤4.5%. The presence of CAD was defined as angiographically detectable atherosclerotic vessel alterations of any degree. Exercise electrocardiography and myocardial perfusion imaging had been performed on an outpatient basis. Statistical analysis was conducted by analysis of variance and Mantel-Haenszel chi-square test. Patients with CAD (n = 101) had a significantly lower FMD% than patients without CAD (n = 21; 3.7% ± 4.1% vs 7.01% ± 3.5%, P < .001). A sensitivity of 71%, a specificity of 81% with a positive predictive value of 0.95 (72 of 76), and a negative predictive value of 0.41 (17 of 46) was calculated. In comparison to angina pectoris (sensitivity 95%, specificity 47.6%), exercise electrocardiography (sensitivity 82.4%, specificity 57.1%) and myocardial perfusion imaging (sensitivity in our study group 100%) had the best specificity, and a high sensitivity for FMD% was found. Conclusions The determination of ED was found to be a sensitive and specific screening test to predict the presence of CAD. Because this is a noninvasive, nonradioactive, and cost-effective approach, it warrants further evaluation to determine its value in daily clinical practice as an additional screening test in the diagnosis of CAD. (Am Heart J 1999;138:731-9.)