Soluble interleukin-2 receptor (sIL-2R) was measured in the plasma of 31 patients with idiopathic thrombocytopenic purpura (ITP) and 22 normal controls. When thrombocytopenia persisted longer than 6 months, the diagnosis of chronic ITP was made. Twenty patients had acute ITP, 11 patients had chronic ITP, and all patients received high-dose methylprednisolone (HDMP) (30 mg/kg/d for 3 days, 20 mg/kg/d for 4 days). The sIL-2R levels of the patients were determined before being giving HDMP and 14 days after the end of HDMP therapy. Platelet counts were determined before administration of HDMP, one day after the end of HDMP therapy, and once every 28 days for 7 months thereafter.