‘Stress thrombocytes’, i.e. large and presumably hyperfunctioning platelets, is a well-known characteristic of patients with idiopathic thrombocytopenic purpura (ITP). Therefore, despite what may be severe thrombocytopenia these patients generally do not suffer from severe life-threatening hemorrhage. The plasma level of soluble P-selectin (sP-selectin) is a valuable marker reflecting platelet activation. Available data suggest that interleukin-6 (IL-6) may contribute to the regulation of megakaryocytopoiesis and platelet activity. The purpose of this study is to investigate the status and kinetics of IL-6 and selectins, which are involved in the platelet function, production, and immunologic functions, during the clinical course of ITP, that may be helpful for understanding the biology of the disease. Twenty-two ITP patients were studied prospectively in the course of their disease. Sixteen, 8 and 6 patients were available after platelet recovery, relapse and splenectomy, respectively. Fifteen healthy persons served as a control group. Higher levels of both sP-selectin and IL-6 were observed in all clinical stages of disease compared to the control group. However, more prominent elevations were present during active stages of ITP, i.e. pretreatment (p < 0.001 vs. control group for both sP-selectin and IL-6) and relapse periods (p < 0.001 vs. control group for both sP-selectin and IL-6). Pretreatment soluble L-selectin and soluble E-selectin levels were not different from the controls. Both sP-selectin (r = –0.32, p = 0.019) and IL-6 (r = –0.41, p = 0.002) levels inversely correlated with platelet count during disease course. There was a positive correlation between the sL-selectin level and leukocyte count (r = 0.60, p < 0.001). These results suggest that residual platelets are activated in ITP, which offers a relatively benign clinical course compared to other thrombocytopenias. High IL-6 concentration during thrombocytopenia may be involved in compensatory megakaryocytopoiesis and augmented ‘residual platelet’ functions in ITP.