Background: Immune dysregulation, polyendocrinopathy, enteropathy, X-linked (IPEX) results in systemic autoimmunity from birth and can be caused by mutations in the transcription factor forkhead box P3 (FOXP3). Objective: To determine if Foxp3 is required for the generation of IL-10-expressing T regulatory cells. Methods: CD4 lymphocytes were isolated from patients with IPEX-like syndromes and activated with antibodies to CD3 and CD46 to generate IL-10-expressing T regulatory cells. Results: We describe a patient with clinical manifestations of IPEX that had a normal Foxp3 gene, but who had CD25 deficiency due to autosomal recessive mutations in this gene. This patient exhibited defective IL-10 expression from CD4 lymphocytes, whereas a Foxp3-deficient patient expressed normal levels of IL-10. Conclusion: These data show that CD25 deficiency results in an IPEX-like syndrome and suggests that although Foxp3 is not required for normal IL-10 expression by human CD4 lymphocytes, CD25 expression is important. Clinical implications: Any patient with features of IPEX but with a normal Foxp3 gene should be screened for mutations in the IL-2 receptor subunit CD25.