Thrombopoietin (Tpo) and stem cell factor (SCF) are growth factors for megakaryocyte progenitor cells and can also modulate platelet function. We have characterized variations in serum levels of these two cytokines in acute leukemia patients undergoing intensive chemotherapy. Compared with healthy controls, serum Tpo levels were significantly increased prior to consolidation chemotherapy, and serum levels were correlated to peripheral blood platelet counts. Serum Tpo levels increased when the patients developed chemotherapy-induced cytopenia, and a further increase was observed during complicating bacterial infections. In contrast to Tpo, SCF serum levels in leukemia patients did not differ from healthy controls neither before chemotherapy nor during the period of chemotherapy-induced cytopenia. Serum levels of Tpo (and possibly SCF) may influence thrombopoiesis and/or platelet functions in patients undergoing intensive chemotherapy for acute leukemia.