B ronchiolitis obliterans syndrome (BOS) is the major limitation to survival after lung transplantation. Acute rejection, its main risk factor, is characterized by perivascular/bronchiolar leukocyte infiltration. BOS is characterized by persistent peribronchiolar leukocyte recruitment leading to airway fibrosis and obliteration. The specific mechanism(s) by which these leukocytes are recruited are unknown. Because MCP-1, acting through its receptor CCR2, is a potent mononuclear cell chemoattractant, we hypothesized that expression of this chemokine during an allogeneic-response promotes persistent recruitment of leukocytes and, ultimately, rejection. We found that elevated levels of biologically active MCP-1 in human bronchial lavage fluid (BALF) were associated with the continuum from acute to chronic allograft rejection. Translational studies in a murine model of BOS demonstrated increased MCP-1 expression paralleling mononuclear cell recruitment and CCR2 expression. Loss of MCP-1/CCR2 signaling, as seen in CCR2^–/– mice or in WT mice treated with neutralizing antibodies to MCP-1, significantly reduced recruitment of mononuclear phagocytes following tracheal transplantation and led to attenuation of BOS. Lymphocyte infiltration was not reduced under these conditions. We suggest that MCP-1/CCR2 signaling plays an important role in recruitment of mononuclear phagocytes, a pivotal event in the pathogenesis of BOS.