This is a prospective study to establish the normal ranges of the proximal left (LCA) and right (RCA) coronary artery diameters in normal children. Echocardiographic measurements of the internal diameters of the LCA, RCA, and the aortic annulus (AoA) were performed on 390 Asians with normal hearts, between the ages of 2 months to 8 years. The maximal diameters of the LCA and RCA in diastole were measured at predetermined sites. The LCA and RCA diameters correlated linearly with age, height, weight, body surface area, as well as the AoA (Pearson's R > 0.8, p <0.005). Regression equations and z-score graphs were constructed. The coronary-aorta index (coronary artery to aortic annulus ratio) falls within a narrow range- LCA/AoA = 0.15 +/- 0.02 (range 0.09-0.21), RCA/AoA = 0.13 +/- 0.02 (range 0.09-0.20). This is independent of age, sex, weight, height, and body surface area. We have established reference ranges for proximal coronary artery diameters in normal children. The regression equations and z-score graphs for the LCA and RCA provide objective determination of coronary size abnormalities. The coronary-aorta index can serve as a quick guide to detect coronary dilatation.