Immune dysregulation, polyendocrinopathy, enteropathy, X-linked (IPEX) syndrome is a rare, fatal autoimmune disorder caused by mutations in the FOXP3 gene leading to the disruption of signaling pathways involved in regulatory T-lymphocyte function. Lifelong multiagent immunosuppression is necessary to control debilitating autoimmune manifestations such as colitis and food allergies. Allogeneic hematopoietic stem cell transplantation (HSCT) can restore T-cell regulatory function but has been previously associated with poor outcome. We describe successful HSCT in 4 patients with IPEX syndrome using a novel reduced-intensity conditioning regimen that resulted in stable donor engraftment, reconstitution of FOXP3⁺ T regulatory CD4⁺ cells, and amelioration of gastrointestinal symptoms.