Myelofibrosis is usually observed in myeloproliferative disorders, such as chronic myeloid leukemia. However, there are only a few reports showing an association between T-cell lymphoma and myelofibrosis. We report a case of peripheral T-cell lymphoma, unspecified (diffuse large cell) type, involving the bone marrow that was associated with severe myelofibrosis. In the present case, the plasma concentration of transforming growth factor-beta1 (TGF-beta1) was increased to 8.95 ng/ml (normal range: 1.56-3.24 ng/ml). No lymphadenopathy or skin lesions were observed during the entire clinical course. Although the mechanism of secondary myelofibrosis is still unclear, elevated plasma TGF-beta1 might be involved in the pathogenesis of bone marrow fibrosis in the present case.