Aim To examine the protective effect of extracorporeal counterpulsation on endothelial dysfunction in patients with syndrome X. Methods Thirty patients with syndrome X were divided into extracorporeal counterpulsation group （n15） and routine treatment group （n15） in a randomized or single blind trial. Before and after the treatment, treatment for 3 weeks, the level of plasma endothelin-1 （ET-1） was assayed, the flow-mediated dilatation （FMD） function in brachial artery was measured by high-resolution ultrasound, and the normal group was without any treatment. Results In patients with syndrome X, the FMD function in artery was much reduced, the level of plasma ET-1 was greatly increased compared with healthy people. Before treatment the FMD function in extracorporeal counterpulsation group was not different from that of routine treatment group. After treatment the FMD function in extracorporeal counterpulsation group was markedly increased and plasma ET-1 was decreased compared with routine treatment group. Conclusion In patients with syndrome X there are endothelial lesion and endothelium-dependent vasodilatation dysfunction. Extracorporeal counterpulsation is very effective in the increasing of FMD function and relieving of myocardial ischemia in patients with syndrome X.