The proteins β-thromboglobulin, platelet factor 4 and thrombospondin are stored in platelet α-granules and released from the platelet by the release reaction. The assays of these proteins were studied in patients with type I diabetes mellitus (n = 30) and a healthy control group (n = 15). Platelet factor 4 and β-thromboglobulin levels were not significantly different in both groups but thrombospondin concentrations in diabetic patients were significantly higher than those of the control group (136.6 ± 14.2 ng/ml vs 91.2 ± 14.3 ng/ml, P < 0.05). When the diabetic patients were divided into those with or without complications, the diabetic patients with complications (n = 11) had significantly elevated plasma thrombospondin concentrations compared with the control group (150.4 ± 23.7 ng/ml vs 91.2 ± 14.3 ng/ml, P < 0.05), while thrombospondin concentrations in the control group were not statistically different from the diabetic patients without complications. Plasma β-thromboglobulin and platelet factor 4 levels were not significantly different between the diabetic and the control group. It is suggested that thrombospondin may be a convenient marker of in vivo platelet release reaction.