We investigated coronary segmental responses to intracoronary acetylcholine (ACh) in 19 patients with hypercholesterolemia and 18 patients with normal cholesterol levels. All patients had atypical chest pain and angiographically normal coronary arteries. After baseline angiography, ACh (3 and 30μg/min) was infused into the left coronary artery, followed by infusion of nitroglycerin. Percent changes in diameter of the proximal, middle, and distal segments of the left coronary arteries were measured by quantitative angiography. In the normocholesterolemic group, 3μg/min of ACh produced significant coronary vasodilation in the distal segments (+8.2±2.6%, p<0.005), while 30 μg/min did not cause any changes. In the hypercholesterolemic group, 30μg/min of ACh caused significant coronary vasoconstriction in the middle and distal segments (-7.2±1.9% and -6.2±1.9%, p<0.001 and p<0.01, respectively), while 3μg/min caused no changes. In each group, vasodilator responses to nitroglycerin in the middle and distal segments were significantly greater than those in the proximal segments (p<0.001). Our results suggest that impaired endothelial function may be evaluated more effectively in the distal coronary segments in patients in the early stage of epicardial coronary atherosclerosis attributable to hypercholesterolemia. J Atheroscler Thromb, 1996 ; 3 : 39-44.