Objectives We performed this study to determine the optimal intravascular ultrasound (IVUS) criteria and to evaluate their accuracy for defining the functional significance of intermediate coronary stenoses in different locations of the coronary tree. Background Presence of myocardial ischemia is the most important prognostic factor in patients with coronary artery disease and is determined by both the lesion severity and the amount of myocardium supplied. Methods IVUS and fractional flow reserve (FFR) measurements were performed in 267 intermediate lesions located at the proximal or mid part of major epicardial coronary arteries. Optimal IVUS criteria and their diagnostic accuracy for functionally significant stenoses (FFR <0.8) were assessed. Results FFR was <0.8 in 88 lesions (33%). The determinants of FFR were minimum lumen area (MLA) and lesion location. The diagnostic accuracy of MLA was highly variable according to the location of lesions. The best cutoff value of MLA to define the functional significance was 3.0 mm2 (area under the curve AUC: 0.81, 95% confidence interval CI: 0.68 to 0.91) for proximal left anterior descending artery (LAD) lesions and 2.75 mm2 for mid-LAD lesions located before the second diagonal branch (AUC: 0.76, 95% CI: 0.66 to 0.84). However, the appropriate MLA to predict the functional significance of lesions could not be found in other segments. Conclusions When IVUS parameters are used to determine the functional significance of lesions in patients with intermediate coronary artery stenoses, different criteria should be used according to lesion location. In segments or vessels with anatomic variations, IVUS cannot be used for functional assessment of a stenosis. (Comparison of Fractional Flow Reserve and Intravascular Ultrasound; NCT01133015 )