Although bone marrow fibrosis is a lethal condition, its underlying mechanism is not fully understood. This study aimed to investigate the pathogenesis of fibrosis in the bone marrow through histologic examination of mast cell infiltration and the expression of fibrosis-associated cytokines. We analyzed 22 bone marrows with fibrosis (8 primary myelofibrosis [PMF], 5 post-essential thrombocythemia [ET], myelofibrosis, and 9 myelodysplastic syndrome [MDS] with bone marrow fibrosis [BMF]). Immunohistochemical and immunofluorescence stainings were performed using anti mast cell tryptase, interleukin (IL) 13, transforming growth factor beta(TOF-beta), CD34, and CD42b antibodies. The number of mast cells in bone marrows with fibrosis was significantly higher than that in controls (P <.0001 for all cases with fibrosis versus control, P =.0470 for PMF versus control, P <.0001 post-ET myelofibrosis versus control, and P =.0005 for MDS with BMF versus control). Moreover, bone marrows with higher fibrotic grades exhibited greater amounts of infiltrating mast cells. Mast cells were positive for TGF-beta and IL-13 in bone marrows with fibrosis of all 3 groups. Megakaryocytes were negative for TGF-beta in post-ET and MDS with BMF, but some megakaryocytes in PMF were weakly positive for TGF-beta. Megakaryocytes were negative for IL-13 in all 3 groups. Blasts were negative for both TGF-beta and IL-13 in all 3 groups. Thus, TOF-beta- and IL-13-producing mast cells might be key players in the development of BMF. Therefore, mast cells could be potential therapeutic targets for the treatment of BMF. (C) 2017 Elsevier Inc. All rights reserved.