Lymphopenia and restricted T cell repertoires in humans are often associated with severe eosinophilic disease and a T cell Th2 bias. To examine the pathogenesis of this phenomenon, C57BL/6 Rag2−/− mice received limited (3 × 10⁴) or large (2 × 10⁶) numbers of CD4 T cells. Three to 5 months after transfer, mice that had received 3 × 10⁴ T cells, but not those that received 2 × 10⁶, developed fulminant macrophage pneumonia with eosinophilia, Ym1 deposition, and methacholine-induced airway hyperresponsiveness, as well as eosinophilic gastritis; esophagitis and other organ damage occurred in some cases. Donor cells were enriched for IL-4, IL-5, and IL-13 producers. When 3 × 10⁴ cells were transferred into CD3ε−/− hosts, the mice developed strikingly elevated serum IgE. Prior transfer of 3 × 10⁵ CD25+ CD4 T cells into Rag2−/− recipients prevented disease upon subsequent transfer of CD25− CD4 T cells, whereas 3 × 10⁴ regulatory T cells (Tregs) did not, despite the fact that there were equal total numbers of Tregs in the host at the time of transfer of CD25− CD4 T cells. Limited repertoire complexity of Tregs may lead to a failure to control induction of immunopathologic responses, and limitation in repertoire complexity of conventional cells may be responsible for the Th2 phenotype.