We studied platelet function and antithrombrin III levels in 30 insulin-dependent diabetic children with no clinically evident vascular complications. 9 were in-patients and 21 were out-patients. The disease had been discovered within the previous 10 years. 25 control subjects of comparable age and body weight were studied simultaneously. Template bleeding time, threshold concentrations of ADP or adrenaline required to induce irreversible platelet aggregation and plasma antiheparin activity (platelet factor 4) did not differ significantly in control and patient groups. In contrast, the immunological levels of plasma antithrombin III were significantly higher in the diabetic group. These results suggest that diabetic children, with no clinical signs of microangiopathy, show no laboratory changes suggesting increased platelet function. The unexpected increase in the antithrombin III level could reflect a very early defense mechanism against activation of the blood clotting system.