Plasma Interleukin-6 (IL-6) level was measured in patients with idiopathic thrombocytopenic purpura (ITP), systemic lupus erythematosus (SLE), rheumatoid arthritis and aplastic anemia. Increase in the plasma level of IL-6 was observed in patients with ITP and SLE. The plasma IL-6 level decreased with progression of the treatment for ITP, and it showed weak negative correlations with the platelet count at the onset of ITP. The increases in the plasma IL-6 level suggest the involvement of activation of the immune system in the pathogenesis of ITP.