Rationale: Arhgef1 is an intracellular protein expressed by hematopoietic cells that regulates signaling by both G-protein coupled receptors and RhoA and, consequently, is required for appropriate migration and adhesion of diverse leukocyte populations. Objective: To evaluate a possible contribution for Arhgef1 in the development of airway inflammation and airway hyperreactivity. Methods: Arhgef1-deficient (Arhgef1^-/-) and wild-type mice were sensitized and airway challenged followed by measurement of airway responsiveness to inhaled methacholine. Inflammation was assessed by several parameters that included flow cytometric analysis and histology. Arhgef1-deficient recipients were reconstituted with wild type T lymphocytes prior to sensitization and challenge and again measured for airway responsiveness and inflammation. Cytokine production in response to specific antigen was measured in cultures of isolated leukocytes from lung and spleen and compared to the levels generated in lung and spleen explant cultures. Results: Arhgef1^-/- mice display significantly reduced airway hyperreactivity, Th2 cytokine production, and lung inflammation, despite intact systemic immunity. After airway challenge of Arhgef1^-/- mice, antigen-specific T cells were present in mutant lungs but found to interact with CD11c⁺ cells at a significantly reduced frequency. Adoptive transfer of wild type T cells into Arhgef1^-/- mice restored airway hyperreactivity and inflammation. Conclusions: These data demonstrate that T cells depend on Arhgef1 to promote lung inflammation. Moreover, a deficiency in Arhgef1 results in reduced T cell-CD11c⁺ antigen presenting cell interaction and likely underscores the inability of Arhgef1^-/- mice to mount an adaptive immune response to airway challenge.