Thrombopoietin (TPO) regulates megakaryocytopoiesis and platelet production in vivo and in vitro. Exogenous overexpression of TPO in vivo by viral-mediated gene transfer induced bone marrow (BM) fibrosis and osteosclerosis. On the other hand, transgenic mice (Tg) overexpressing TPO using a liver-specific apolipoprotein E (Apo-E) promoter did not exhibit myelofibrosis or osteosclerosis. These discrepancies in phenotype are not fully understood. Then we have investigated the consequences of long-term in vivo overexpression of TPO in a mouse model. Murine TPO Tg mice driven by the IgH promoter were generated. The number of platelets and neutrophils in peripheral blood, and the number of megakaryocytes and granulocytic immature cells in the BM was elevated, together with the number of progenitor cells for megakaryocyte and myeloid cells. TPO Tg mice demonstrated anemia but the number of progenitor cells for the erythrocyte was increased. TPO Tg mice developed myelofibrosis and osteosclerosis as they aged with extramedullary hematopoiesis in the spleen. As plasma transforming growth factors (TGF)-beta1 and osteoprotegerin (OPG) levels were higher in TPO Tg mice than in wild-type mice, the development of myelofibrosis and osteosclerosis depends on local TPO levels in BM and might be due to elevated TGF-beta1 and OPG.