To evaluate the effect of ramipril, an ACE inhibitor, on flow-mediated dilation (FMD) of the brachial artery (an index of endothelium-dependent vasodilation), in patients with hypertension. The study population included 96 Chinese patients with newly diagnosed essential hypertension (52 men and 44 women; mean age 56 ± 7 years) and 30 normotensive subjects (16 men and 14 women; mean age 53± 9 years). The 96 patients with essential hypertension were randomised to receive 3 months of oral ramipril 5mg once daily (ramipril group, 49 cases) or oral nitrendipine 10mg twice daily (nitrendipine group, 47 cases). Baseline diameter, reactive hyperaemia (RH), FMD and nitroglycerin-induced dilation (NTG dilation) were evaluated with the non-invasive ultrasound method before and after treatment in the two hypertensive groups. FMD was significantly lower in the two hypertensive groups than in the control group before treatment and improved significantly in the ramipril group after treatment compared with before treatment (5.1± 2.2 versus 1.8 ± 1.2, p < 0.01). The baseline diameters of the brachial arteries, RH and NTG dilation showed no difference between the control group and the two hypertensive groups before treatment and remained unchanged after treatment in both the ramipril group and the nitrendipine group. Blood pressure was reduced to the same extent in the two hypertensive groups (p > 0.05) and the changes in mean blood pressure did not correlate with the changes in FMD in the ramipril group (r = 0.22). In addition to blood pressure control, ramipril can improve endothelial dysfunction in patients with essential hypertension.