Obesity, a major health problem in industrialized societies, is associated with a high incidence of cardiovascular complications such as hypertension, ischemic heart disease and stroke. However, the underlying mechanism relating obesity and these cardiovascular events is not clear. In lean subjects even slight elevations in plasma insulin concentration exert marked effects on the cardiovascular system, and these effects are directly related to insulin (rather than to insulin-induced stimulation of intermediary metabolism). Moreover, insulin's vascular effects are mediated both by the sympathetic nervous system and the L-arginine nitric oxide pathway. Obesity is characterized by sustained sympathetic activation (possibly related to chronic hyperinsulinemia) and an impaired vasodilator responsiveness to insulin. Although, undoubtedly many factors contribute to the increased incidence of cardiovascular complications in overweight subjects, sympathetic activation could be one important mechanism and either trigger acute events or--possibly in conjunction with an impairment in insulin-induced vasodilation--contribute to sustained elevation of arterial pressure.