AIM: The purpose of this study was to determine whether there are abnormalities in flow-mediated large vessel relaxation in patients with congestive heart failure (CHF). METHODS: The radial arterial diameter and flow responses upon the release of 10 min of forearm arterial occlusion (reactive hyperemia) were measured with ultrasound and Doppler devices. RESULTS: In patients with CHF there was a 26% reduction in peak blood flow (P = 0.09) compared to age-matched controls. However, the increase in arterial diameter that followed the peak blood flow was reduced by 49% in CHF (P < 0.01). CONCLUSIONS: The causes of the abnormal flow-mediated large artery relaxation in CHF are unclear; both structural and endothelial abnormalities may contribute.