Acetylcholine stimulates endothelial synthesis of nitric oxide from L-arginine. To investigate the influence of sex on endothelial function, we measured vasodilator responses to brachial artery administration of acetylcholine in hypercholesterolaemic and control men and women. Mean response to acetylcholine was impaired (55% of that in controls at 15 μg/min) in hypercholesterolaemic men but not in hypercholesterolaemic women. L-arginine normalised responses to acetylcholine in hypercholesterolaemic men, but had similar effects in hypercholesterolaemic and control women. These results suggest that women are protected against adverse effects of hypercholesterolaemia on the L-arginine/nitric-oxide pathway.