Obesity is associated with impaired endothelial function, and this may lead to increased cardiovascular risk. To gain insight into the beneficial effects of diet-induced weight loss on endothelial function, endothelium-dependent, flow-mediated dilation (FMD) of the brachial artery and several metabolic and inflammatory markers were assessed in 40 obese women (BMI 34.9 ± 4.88 kg/m²) at baseline, after the 1st week and after 5 months on a low-calorie diet of 5.0 MJ/day. Twenty lean women served as controls. At entry, the obese women had a lower FMD than the lean women (7.7 ± 1.8 vs. 11.5 ± 4.2%, p < 0.001). After 1 week of the intervention and 4% reduction of BMI, FMD improved by 22% (p = 0.005), and a decrease in circulating triglycerides, insulin, leptin, tissue type plasminogen activator and its inhibitor, von Willebrand factor, C-reactive protein and tumor necrosis factor receptor 1 was observed. Improvement of FMD was associated only with a decrease in BMI (r = 0.39, p = 0.03). Twenty-two women completed the weight reduction program and reduced their BMI by 16%. FMD was further improved by 64% (to 12.4 ± 5.3%, p = 0.001) and became comparable to that of lean women. None of the significant changes in the observed parameters was associated with improvement of FMD at the end of the program. Improvements in obesity-related endothelial dysfunction began in the 1st week of dieting and continued during the following months of this simple non-pharmacological lifestyle modification to reach normalisation of endothelial function. The favourable effect of dieting on endothelial function is independent of the accompanying improvement of classical risk factors.