In view of the tendency toward vascular complications in diabetes mellitus, we studied platelet aggregation and plasma beta-thromboglobulin (beta-TG) levels in 35 healthy controls, 25 non-diabetic patients with ischemic heart disease (IHD) and 85 diabetic patients. Blood platelets from diabetic patients showed no significant hyperaggregation induced by ADP, collagen or epinephrine, as compared with controls and non-diabetic patients with IHD, nor could be demonstrated significant differences in platelet aggregation between diabetics with diabetic microangiopathies and those without diabetic vascular complications. Significantly high levels of plasma beta-TG were observed in diabetics in comparison with those in controls and non-diabetic patients with IHD. Patients with diabetic microangiopathy had more significantly elevated beta-TG levels than diabetics without diabetic microangiopathies. Diabetics without diabetic microangiopathy had similar levels of beta-TG to those of controls and non-diabetic patients with IHD. High levels of plasma beta-TG in diabetics with diabetic microangiopathy seem to indicate a platelet hyperfunction in vivo due to diabetic vascular complications.