Acute lung injury (ALI) causes significant morbidity and mortality. Fibroproliferation in ALI results in worse outcomes, but the mechanisms governing fibroproliferation remain poorly understood. Regulatory T cells (Tregs) are important in lung injury resolution. Their role in fibroproliferation is unknown. We sought to identify the role of Tregs in ALI fibroproliferation, using a murine model of lung injury. Wild-type (WT) and lymphocyte-deficientRag-1-/-mice received intratracheal LPS. Fibroproliferation was characterized by histology and the measurement of lung collagen. Lung fibrocytes were measured by flow cytometry. To dissect the role of Tregs in fibroproliferation,Rag-1-/-mice received CD4+CD25+ (Tregs) or CD4+CD25- Tcells (non-Tregs) at the time of LPS injury. To define the role of the chemokine (C-X-C motif) ligand 12 (CXCL12)-CXCR4 pathway in ALI fibroproliferation,Rag-1-/-mice were treated with the CXCR4 antagonist AMD3100 to block fibrocyte recruitment. WT andRag-1-/-mice demonstrated significant collagen deposition on Day 3 after LPS. WT mice exhibited the clearance of collagen, butRag-1-/-mice developed persistent fibrosis. This fibrosis was mediated by the sustained epithelial expression of CXCL12 (or stromal cell-derived factor 1 [SDF-1]) that led to increased fibrocyte recruitment. The adoptive transfer of Tregs resolved fibroproliferation by decreasing CXCL12 expression and subsequent fibrocyte recruitment. Blockade of the CXCL12-CXCR4 axis with AMD3100 also decreased lung fibrocytes and fibroproliferation. These results indicate a central role for Tregs in the resolution of ALI fibroproliferation by reducing fibrocyte recruitment along the CXCL12-CXCR4 axis. A dissection of the role of Tregs in ALI fibroproliferation may inform the design of new therapeutic tools for patients with ALI.