Patients with immune thrombocytopenic purpura (ITP) rarely suffer life-threatening haemorrhages despite significant thrombocytopenia, probably because large numbers of hyperfunctioning platelets are present. Thrombospondin is a platelet α-granule protein and its plasma level may reflect platelet activation. We assessed circulating thrombospondin levels in 12 newly diagnosed ITP patients (one man; 11 women, aged 36 ± 16 years) before they were treated for ITP. Twelve healthy people (four men; eight women, aged 31 ± 11 years) acted as controls. Plasma thrombospondin concentrations were measured using enzyme-linked immunoassays. Thrombospondin concentrations tended to be higher, despite thrombocytopenia, in ITP patients (158.8 ± 28.2 ng/ml) compared with controls (120.7 ± 18.2 ng/ml). The difference was not statistically significant, but the relatively high circulating thrombospondin concentrations we observed suggest that residual platelets could be activated in ITP, thus indicating a more benign clinical course compared with aplastic thrombocytopenia.