The mechanisms that lead to reticulin fibrosis of bone marrow (BM) in hairy cell leukemia (HCL) are not fully understood. We therefore investigated the involvement of TGF-β1, a potent fibrogenic cytokine, in this process. Immunoassays revealed that TGF-β1 is present at higher concentrations in BM, serum, and plasma of HCL patients in comparison with healthy donors (P < 0.001). RT-PCR and immunofluorescence studies showed that TGF-β1 is overexpressed at the mRNA and protein levels in peripheral blood, spleen, and BM mononuclear cells and that hairy cells (HCs) are the main source of TGF-β1. Active TGF-β1 correlated significantly with grades of BM fibrosis, infiltration with HCs, and serum procollagen type III aminoterminal propeptide (PIIINP). Ex vivo studies demonstrated that TGF-β1 significantly enhances the production and deposition of reticulin and collagen fibers by BM fibroblasts. In addition, BM plasma of HCL patients increased the synthesis of type I and type III procollagens, the main components of reticulin fibers, at the mRNA and protein levels. This fibrogenic activity of BM plasma was abolished by neutralizing anti–TGF-β1 antibodies. These results show, for the first time to our knowledge, that TGF-β1 is highly expressed in HCs and is directly involved in the pathogenesis of BM reticulin fibrosis in HCL.