There is a growing body of evidence suggesting that numerous pathological conditions are associated with increased vascular production of reactive oxygen species. This form of vascular oxidant stress and particularly interactions between NO and oxygen-derived radicals represent a common pathological mechanism present in many so-called risk factors for atherosclerosis. Furthermore, reactive oxygen species seem to serve important cellular signalling mechanisms responsible for many of the features of vascular lesion formation. The mechanisms whereby vascular cells produce reactive oxygen species are only presently coming to light, and almost certainly will prove to be a focus for future therapies.