Plasma concentrations of the most potent megakaryocytopoietic cytokines, thrombopoietin (TPO) and interleukin-6 (IL-6), and the platelet activation marker P-selectin were evaluated in 24 patients with autoimmune thrombocytopenic purpura (ATP) who responded to conventional steroid treatment, at diagnosis and after steroid-induced recovery. Baseline TPO concentration (median [interquartile range] = 0 [17.52] pg/ml) was significantly decreased and IL-6 (38 [19.75] pg/ml) and P-selectin (485 [393.75] ng/ml) were significantly elevated compared with healthy subjects (100  pg/ml, 8  pg/ml and 166  ng/ml, respectively). Following steroid treatment, all values approached normal, i.e., TPO (20 [18.75] pg/ml) was increased and IL-6 (19.5  pg/ml) and P-selectin (248 [172.5] ng/ml) were decreased, significantly. The decrease of TPO in ATP is suggested to occur due to increased megakaryocyte mass and, consequently, TPO clearance. The non-lineage-specific cytokine IL-6 may be elevated to compensate for megakaryocytopoiesis/thrombopoiesis. The elevation of P-selectin may reflect compensatory platelet hyperactivation; however, this molecule also might be a marker of platelet destruction.