Romiplostim-induced MF in a mouse model demonstrated extensive bone marrow (BM) CHI3L1 mRNA expression, which was reversed by clodronate treatment. A culture assay revealed that high CHI3L1 concentrations promoted extracellular matrix production by fibroblast cell lines, and that a CHI3L1-neutralizing antibody abrogated this effect. These results indicate the importance of CHI3L1 in the association between fibrocytes and fibroblasts in MF and could be a focus for future treatment.