Background Vasodilator response to acetylcholine (ACh) (ie, endothelium-dependent dilation) is impaired in the peripheral and coronary circulation of patients with coronary risk factors and coronary artery disease (CAD). There is a close relationship of vasodilator response to ACh in both the coronary artery (CA) and the brachial artery (BA), but the comparative prognostic importance of these responses has not been fully investigated in relatively low-risk suspected CAD. Methods and Results The flow responses of both the CA and BA were measured in 70 patients with suspected CAD, excluding patients with triple-vessel disease and known peripheral or cerebrovascular disorders. A Doppler guidewire was placed into a major branch of the CA and a proximal portion of the left BA. ACh was infused at 10^-8, 10^-7 and 10^-6 mol/L for 3 min into the CA and at 7.5, 15, and 30 μg/min for 5 min into BA. The flow response was obtained by multiplying the average peak velocity by the cross-sectional area from quantitative angiography. Vasodilator response to ACh was assessed by the ratio of ACh-induced flow/baseline flow, expressed as coronary blood flow index (CBFI) or brachial blood flow index (BBFI). There were 39 CAD patients (61±8 years old) and 31 normal coronary patients (NL, 58±11 years old) who were followed up for 53±17 months. Eleven patients had coronary events (CE) during this period: 1 case of nonfatal myocardial infarction and 10 cases of unstable angina. A strong correlation between CBFI and BBFI was observed at middle-and high-doses of ACh (r=0.72, p<0.0001, 15 μg/min vs 10^-7 mol/L; r=0.76, p<0.0001, 30 μg/min vs 10^-6 mol/L). Kaplan-Meier analysis, using the best cut-off values obtained from receiver-operating characteristic curves for CE, revealed that both CBFI and CAFI were significant predictors for CE. Conclusions The BA vasodilator response to optimal ACh dosage can be used as a surrogate prognostic predictor for coronary endothelial function tests in patients with suspected CAD. (Circ J 2006; 70: 49 - 56)