The scurfy mutant mouse is the genetic and phenotypic equivalent of the single-gene human autoimmune disease immunodysregulation, polyendocrinopathy, enteropathy, X-linked (IPEX). The scurfy mutation disrupts the Foxp3 gene, a putative master switch for T regulatory cell development. Bone marrow transplant without conditioning was previously reported to be ineffective in scurfy mice, yet clinical remission occurs in transplanted human IPEX patients despite limited donor engraftment. In view of this contradiction, we sought to validate scurfy as a model for studying the pathogenesis and treatment of human IPEX, in particular the phenomenon of dominant immune regulation. One half of scurfy mice given bone marrow transplants after sublethal irradiation recovered and survived long-term with donor chimerism ranging from 1·7% to 50%. Early transfer of 2 × 107 normal T cell-enriched splenocytes also prevented or limited disease and permitted long-term survival. Donor T cells in rescued mice made up 3–5% of lymphocytes and became highly enriched for CD25+ T cells over time. Transfer of 106 CD4+ CD25+ sorted T cells showed some beneficial effect, while CD4+ CD25- cells did not. Thus, both partial bone marrow transplant and T-enriched splenocyte transfer are effective treatments for scurfy. These results indicate that scurfy results from a lack of cells with dominant immune regulatory capacity, possibly T regulatory cells. The potency of small numbers of normal cells indicates that IPEX may be a feasible target for gene therapy.