Hypercholesterolaemia impairs endothelial function, possibly by interference with the intracellular formation of endothelium-derived relaxing factor from its precursor L-arginine. Whether L-arginine reverses hypercholesterolaemia-induced endothelial dysfunction in the coronary circulation was thus investigated. Epicardial artery cross-sectional area and coronary blood flow velocity were measured in 8 hypercholesterolaemic patients (mean serum cholesterol 7.8 [SE 0.3] mmol/l) and 7 age-matched controls before and after graded intracoronary infusions of the endothelium-dependent agent acetylcholine (0.036, 0.36, 3.6 micrograms/min). The effect of intracoronary infusion of L-arginine (160 mumol/min via the guiding catheter) on these measurements was then examined. In controls, acetylcholine induced a moderate dose-dependent constriction of the epicardial artery segment of the left anterior descending artery and increased coronary blood flow (by 239% [SE 57] at the highest dose). In patients with hypercholesterolaemia, the vasoconstrictive effect of acetylcholine on epicardial segments was similar to that in controls, but the increase in coronary blood flow with acetylcholine was significantly attenuated (highest dose: 61%, p less than 0.02 vs controls). L-arginine restored the acetylcholine-induced increase in blood flow in patients with hypercholesterolaemia (198%  vs baseline) but did not affect coronary blood flow in controls. The findings suggest that hypercholesterolaemia impairs endothelium-dependent dilatation of the coronary microcirculation and that this impairment can be restored by short-term administration of L-arginine. The possibility that L-arginine might form the basis of treatment for coronary endothelial abnormalities induced by hypercholesterolaemia could be worth investigating.