Platelet functions were studied in 10 patients with insulin dependent diabetes mellitus (IDDM) with vasculopathy, and in age and sex-matched normal controls. The patient group at presentation showed increased circulating platelet aggregates, greater platelet factor III availability and increased aggregation of platelets in response to various agonists (shorter latent period, greater rate of aggregation, greater degree of aggregation, p<0.05-<0.001). This platelet hyperreactivity remained unchanged even after normalisation of blood glucose. In contrast, similar platelet hyperreactivity seen by us earlier in patients with uncontrolled IDDM without vasculopathy reversed to normal after metabolic control.In view of the postulated role of hyperreactivity of platelets in the pathogenesis of vasculopathy in IDDM, our results imply that once vasculopathy sets in, a vicious cycle starts between platelet hyperreactivity and vasculopathy. Proper and early treatment of IDDM at a stage when vasculopathy is not present, would therefore be important in reversing the platelet hyper-function and may possibly delay the development of vasculopathy.