Investigations on the platelet function in diabetes mellitus were performed on 28 patients with insulin-dependent diabetes and in 33 healthy controls of similar age. In the diabetic patients it was possible to induce 50% of maximal aggregation by lower concentrations of adenosine diphosphate or arachidonic acid than in the controls. In the presence of N-ethyl maleimide, platelets from diabetic patients produced significantly more malondialdehyde than those from normal controls. After addition of arachidonic acid the platelets from the diabetic patients also synthesized more thromboxane B2. This synthesis of thromboxane was inversely correlated to the minimal concentration of arachidonic acid necessary to induce 50% platelet aggregation. Circulating platelet aggregates were more common in the diabetic patients than in the controls. Plasma levels of β-thromboglobulin and platelet factor 4 were raised in parallel in the diabetic patients and correlated with the increased production of thromboxane B2 by the platelets from the same patients. Platelets from patients with diabetes thus demonstrated signs of hyperreactivity both in vivo and in vitro. This may be of clinical importance for the development of vascular complications in this disease