A patient with idiopathic thrombocytopenic purpura (ITP) developed T‐cell lymphoma while undergoing steroid therapy. We examined the relationship between the patient's serum thrombopoietin (Tpo) level, platelet count, megakaryocyte number and CFU‐Meg number during the second 5 d course of chemotherapy for lymphoma in which megakaryopoiesis switched from ITP phase to amegakaryocytic phase. The patient's platelet count was temporarily elevated but CFU‐Meg numbers were markedly suppressed, and megakaryocyte numbers were decreased in this period, whereas serum Tpo level was not suppressed despite an increased platelet count, indicating that serum Tpo level is mainly regulated by megakaryocyte mass.