Endothelium-derived nitric oxide plays a major role in the regulation of vascular tone and in the maintenance of vascular homeostasis. Endothelial dysfunction with impaired nitric oxide biosynthesis and decreased bioavailability has been implicated in insulin resistance syndrome and Type II (non-insulin-dependent) diabetes mellitus. Nitric oxide is synthesised by nitric oxide synthase. Asymmetric dimethylarginine is a major endogenous nitric oxide synthase inhibitor. Increased circulating asymmetric dimethylarginine was initially found in patients with chronic renal failure and subsequently many other disease states. Increased asymmetric dimethylarginine plasma concentrations could contribute to the development of insulin resistance and coronary heart disease. Understanding of the pathophysiological role of asymmetric dimethylarginine could lead to novel therapies in the prevention of arteriosclerosis and coronary heart disease.