BACKGROUND: Endothelial dysfunction, a pathological feature of obesity, can predict the occurrence of cardiovascular disease. The endothelial function was compared in obese, non-obese, and type 2 diabetic women, and the effect of weight loss on endothelial function in obese premenopausal women was also investigated. METHODS: Twenty type 2 diabetes patients, 35 obese and 20 non-obese non-diabetic subjects were recruited. Both the endothelium-dependent vasodilation (EDV) and endothelium-independent vasodilation (EIV) were measured. The body composition, serum lipid, serum adiponectin and resistin were also measured. Weight loss in obese women was obtained by 6 months of calorific restriction, aerobic exercise and medication (sibutramine or orlistat). RESULTS: EDV was significantly impaired in the type 2 diabetes and obese groups compared to the control group (6.0 +/- 1.3% in diabetes group, 6.7 +/- 3.9% in obese group, 12.4 +/- 4.1% in control group, P < 0.01, respectively). The mean weight loss after 6 months was 8.5 +/- 3.2 kg (P < 0.001) in the obese group. There was a significant increase in EDV after weight loss (from 5.8 +/- 3.5% to 12.3 +/- 3.9%, P < 0.05). There was no change in EIV after weight loss. In addition, weight loss was associated with significant reductions in the levels of high-sensitivity C-reactive protein (hs-CRP) and serum triglyceride (P < 0.05, respectively). However, there were no significant changes in the serum adiponectin and resistin levels after weight loss. CONCLUSIONS: Our data demonstrated that weight loss was associated with improved endothelial function in obese premenopausal women, as assessed by brachial artery EDV and reduced hs-CRP.