BACKGROUND: Crohn disease is an inflammatory bowel disease (IBD) with a complex mode of inheritance. Although nucleotide binding and oligomerization domain containing 2 (NOD2) is the strongest risk factor, the cause of Crohn disease remains unknown in the majority of the cases. X-linked inhibitor of apoptosis (XIAP) deficiency causes X-linked lymphoproliferative syndrome type 2. IBD has been reported in some XIAP-deficient patients. OBJECTIVE: We characterize the IBD affecting a large cohort of patients with mutations in XIAP and examine the possible pathophysiologic mechanisms. METHODS: We performed a phenotypical and histologic analysis of the IBD affecting 17 patients with hemizygous mutations in XIAP, including 3 patients identified by screening 83 patients with pediatric-onset IBD. The X chromosome inactivation was analyzed in female carriers of heterozygous XIAP mutations, including 2 adults with IBD. The functional consequences of XIAP deficiency were analyzed. RESULTS: Clinical presentation and histology of IBD in patients with XIAP deficiency overlapped with those of patients with Crohn disease. The age at onset was variable (from 3 months to 41 years), and IBD was severe and difficult to treat. In 2 patients hematopoietic stem cell transplantation fully restored intestinal homeostasis. Monocytes of patients had impaired NOD2-mediated IL-8 and monocyte chemoattractant protein 1 (MCP-1) production, as well as IL-10, in response to NOD2 and Toll-like receptor 2/4 costimulation. Nucleotide binding and oligomerization domain containing 1 (NOD1)-mediated IL-6 and IL-8 production was defective in fibroblasts from XIAP-deficient patients. The 2 heterozygous female carriers of XIAP mutations with IBD displayed abnormal expression of the XIAP mutated allele, resulting in impaired activation of the NOD2 pathway. CONCLUSION: IBD in patients with XIAP deficiency is similar to Crohn disease and is associated with defective NOD2 function in monocytes. Importantly, we report that it is not restricted to male patients because we identified 2 symptomatic female heterozygous carriers of XIAP mutations.