IL-10 is most commonly recognized as an anti-inflammatory cytokine possessing immunosuppressive effects necessary for regulated resolution of proinflammation. However, its role in the development of fibrosis during inflammatory resolution has not been clear. Few prior studies have linked IL-10 with the inhibition of fibrosis principally on the basis of regulating inflammation thought to be driving fibroproliferation. In contrast, in a model of long-term overexpression of IL-10, we observed marked induction of lung fibrosis in mice. The total cell number retrieved by bronchoalveolar lavage (BAL) increased 10-fold in the IL-10 overexpression (IL-10 OE) mice, with significant infiltration of T and B lymphocytes and collagen-producing cells. The presence of increased fibrocytes, isolated from collagenase-digested lungs, was identified by flow cytometry using dual staining of CD45 and collagen 1. Quantitative PCR analysis on an array of chemokine/chemokine receptor genes showed that receptor CCR2 and its ligand, CCL2, were highly upregulated in IL-10 OE mice, suggesting that IL-10-induced fibrocyte recruitment was CCL2/CCR2 specific. Given the prior association of alternatively activated (M(2)) macrophages with development of fibrosis in other disease states, we also examined the effect of IL-10 OE on the M(2) macrophage axis. We observed significantly increased numbers of M(2) macrophages in both BAL and whole lung tissue from the IL-10 OE mice. Administration of rabbit anti-CCL2 antiserum to IL-10 OE mice for three consecutive weeks significantly decreased fibrosis as evidenced by lung hydroxyproline content, compared with mice that received preimmune rabbit serum. These results indicate that overexpression of IL-10 induces fibrosis, in part, by fibrocyte recruitment and M(2) macrophage activation, and likely in a CCL2/CCR2 axis.