A higher intake of black tea has been associated with lower cardiovascular disease risk. The antioxidant effects of tea polyphenols may enhance endothelial function and thereby reduce the risk of coronary events. The objective of the present study was to determine whether regular ingestion of black tea can improve brachial artery vasodilator function. The effects of regular ingestion of 5 cups per day of black tea for 4 weeks were compared with control conditions (hot water ingestion) in 21 subjects with mild elevations in serum cholesterol or triacylglycerol (triglyceride) concentrations in a parallel designed study. Endothelial function of the brachial artery was assessed ultrasonographically by measurement of post-ischaemic (endothelium-dependent) dilatation of the brachial artery. Endothelium-independent dilatation of the brachial artery was measured following administration of 400 microg of sublingual glyceryl trinitrate. Regular ingestion of black tea resulted in a significant and consistent increase in endothelium-dependent dilatation (2.3%; P=0.008) and in a significant increase in endothelium-independent dilatation (4.2%; P=0.03), compared with ingestion of hot water. These differences remained after adjustment for age, sex and body mass index. These results suggest that one mechanism by which black tea may reduce cardiovascular risk is via improved vasodilator function of conduit arteries.