The authors evaluated endothelial function in patients with MELAS (mitochondrial myopathy, encephalopathy, lactic acidosis, and stroke) by flow-mediated vasodilation (FMD) and found a significant decrease vs controls. Two years of supplementation with oral l-arginine, a nitric oxide precursor, significantly improved endothelial function to control levels and was harmonized with the normalized plasma levels of l-arginine in patients. l-Arginine therapy improved endothelial dysfunction and showed promise in treating strokelike episodes in MELAS.