['There is evidence for increased formation of free radicals in patients with hypertension, raising the possibility that NO is inactivated by free radicals, which impairs coronary endothelial function. Therefore, we tested the hypothesis that the antioxidant vitamin C could improve abnormal endothelial function of coronary arteries in patients with hypertension.', 'In 22 hypertensive patients without relevant coronary artery stenoses, endothelium-dependent vascular responses of the left anterior descending coronary artery (LAD) to acetylcholine (0.01, 0.1, and 1.0 micromol/L) were determined before and immediately after intravenous infusion of 3 g vitamin C (17 patients) or placebo (5 patients). In a subgroup of 10 patients, papaverine-induced flow-dependent vasodilation (FDD) was measured before and after vitamin C (5 patients) or placebo (5 patients) infusion. Segmental responses of the coronary artery luminal area were analyzed with quantitative coronary angiography. Before vitamin C infusion, the mean changes of LAD luminal areas at increasing doses of acetylcholine were -6.1+/-2.2%, -15.2+/-4.9%, and -33.9+/-8.1% (negative numbers symbolize vasoconstriction) and during FDD, 5.4+/-1.0%. The vasoconstrictor response during acetylcholine was reduced and FDD was augmented by vitamin C. After vitamin C infusion, LAD luminal areas changed by -3.2+/-2.3%, -5.8+/-3.6%, and -10.2+/-5.6% (P<.05, acetylcholine) and 17.8+/-2.8% (P<.05, FDD). Doppler flow velocity (during baseline, acetylcholine, and FDD) was not significantly affected by vitamin C.'] Does vitamin C improve endothelial dysfunction of epicardial coronary arteries in hypertensive patients?