The effects of age, atherosclerosis, hypertension, and hypercholesterolemia on vascular function of the coronary circulation were studied by subselective intracoronary infusions of acetylcholine, which releases endothelium-derived relaxing factor, and papaverine, which directly relaxes vascular smooth muscle, in normal patients (n = 18; no risk factors for coronary artery disease), in patients with evidence of early atherosclerosis but normal cholesterol levels and normal blood pressure (n = 12), in patients with hypertension without left ventricular hypertrophy (n = 12), and in patients with hypercholesterolemia (n = 20). Papaverine-induced maximal increases in coronary blood flow were significantly greater in normals, but no differences were noted between the groups of patients with early atherosclerosis, with hypertension, and with hypercholesterolemia. The capacity of the coronary system to increase blood flow in response to acetylcholine was similar in normal and normocholesterolemic patients with epicardial atherosclerosis and/or hypertension but was significantly impaired in patients with hypercholesterolemia, irrespective of evidence of epicardial atherosclerotic lesions. Age (r = -0.62, P < 0.0001) and total serum cholesterol levels (r = -0.70; P < 0.0001) were the only significant independent predictors of a blunted coronary blood flow response to acetylcholine. Thus, hypercholesterolemia and advanced age selectively impair endothelium-mediated relaxation of the coronary microvasculature in response to acetylcholine, whereas endothelial dysfunction is restricted to epicardial arteries in age-matched normocholesterolemic patients with evidence of coronary atherosclerosis and/or hypertension.