S WAP-70–like adapter of T cells (SLAT) is a novel guanine nucleotide exchange factor for Rho GTPases that is upregulated in Th2 cells, but whose physiological function is unclear. We show that SLAT^–/– mice displayed a developmental defect at one of the earliest stages of thymocyte differentiation, the double-negative 1 (DN1) stage, leading to decreased peripheral T cell numbers. SLAT^–/– peripheral CD4⁺ T cells demonstrated impaired TCR/CD28-induced proliferation and IL-2 production, which was rescued by the addition of exogenous IL-2. Importantly, SLAT^–/– mice were grossly impaired in their ability to mount not only Th2, but also Th1-mediated lung inflammatory responses, as evidenced by reduced airway neutrophilia and eosinophilia, respectively. Levels of Th1 and Th2 cytokine in the lungs were also markedly reduced, paralleling the reduction in pulmonary inflammation. This defect in mounting Th1/Th2 responses, which was also evident in vitro, was traced to a severe reduction in Ca²⁺ mobilization from ER stores, which consequently led to defective TCR/CD28-induced translocation of nuclear factor of activated T cells 1/2 (NFATc1/2). Thus, SLAT is required for thymic DN1 cell expansion, T cell activation, and Th1 and Th2 inflammatory responses.