The role of oxidative stress and platelet activation in the development and evolution of diabetic vascular complications is unclear. They can both be the consequence of established vascular disease or a contributing factor to the evolution of atherosclerosis. Free radical generation may both lead to or result from platelet activation, suggesting that oxidative stress and platelet activation may be closely interrelated. Controversial results may partly reflect methodological constraints. Novel techniques for the measurement of in vivo indices of oxidant injury have been developed and will be used in conjunction with pharmacological probes to establish whether oxidative stress is enhanced in diabetes and whether this preceeds the onset of micro and macrovascular disease.