Plasma beta-thromboglobulin, hemoglobin A1c and serum creatinine were measured in 56 diabetic patients. Patients with diabetic complications had a significantly higher beta-thromboglobulin level than the controls, but patients without complications did not differ from the controls. beta-Thromboglobulin correlated positively with serum creatinine and the highest values were encountered in patients with abnormal creatinine concentration. There were, however, patients with normal creatinine but high plasma beta-thromboglobulin levels. In these patients the abnormal beta-thromboglobulin level most probably indicates undue stimulation of platelets. There was no correlation between beta-thromboglobulin and hemoglobin A1c.