Ras plays an essential role in lymphocyte development and function. However, in vivo consequence(s) of regulation of Ras activity by guanosine triphosphatase (GTPase)–activating proteins (GAPs) on lymphocyte development and function are not known. In this study we demonstrate that neurofibromin, the protein encoded by theNF1 tumor suppressor gene functions as a GAP for Ras in T cells. Loss of Nf1 in T cells results in enhanced Ras activation, which is associated with thymic and splenic hyperplasia, and an increase in the absolute number of immature and mature T-cell subsets compared with control mice. Interestingly, in spite of a profound T-cell expansion and higher thymidine incorporation in unstimulated Nf1-deficient T cells, T-cell receptor and interleukin-2 receptor–mediated proliferation of thymocytes and mature T cells was substantially reduced compared with control mice. Collectively, these results identify neurofibromin as a GAP for Ras in T cells for maintaining immune homeostasis in vivo.