Endothelial function is thought to play a central role in the process of atherosclerosis, and in the pathogenesis of arterial hypertension and chronic heart failure. In the last decade a non-invasive ultrasound technique has been developed to measure endothelium-dependent vasodilation of the brachial artery which allows quantification of endothelial function. This method is based upon results in basic research, which show increased endothelial NO-production following the impact of shear forces on the endothelial surface. The resulting vasodilation can be quantified with high resolution ultrasound (7-13 MHz). The advantage of this technique is its non-invasive nature, which allows a risk-free use even in young individuals. Repeated measurements to control the success of medical intervention are also not problematic. Despite the possibility of widespread use because of its deceptively simple protocol, there are numerous factors affecting flow-mediated vascular reactivity. Details of the methods applied, variations in examination procedure, image acquisition, subjects preparation, equipment, training requirements and data interpretation are reviewed in this paper. Clinical studies of cardiovascular disease, risk factor profiles and the prognostic impact of measuring flow-mediated vasodilation are also discussed.