Interferon regulatory factor 5 (IRF5) located on human chromosome 7q32 is associated with many chronic inflammatory disorders. IRF5 is the key regulator of proinflammatory cytokines and type I interferons. We surveyed two cohorts of inflammatory bowel disease (IBD) patients from a North American Consortium. Six single-nucleotide polymorphisms and a 5-base-pair (bp) insertion-deletion (CGGGG indel)polymorphism were investigated. Cytokine secretion was measured in primary lymphocytes after toll-like receptor 9 stimulation. Two-marker haplotypes containing the pairs (rs4728142-CGGGG indel) and (CGGGG indel-rs7808907) were associated with IBD protection (P=2.89 × 10(-6), P=9.32 × 10(-4) (non-Jewish ancestry) and P=4.68 × 10(-8), P=2.50 × 10(-8) (Jewish ancestry)) and IBD risk (P=0.004, P=0.003 (Jewish ancestry), respectively. IRF5 polymorphisms were risk factors for IBD in a single cohort. Interleukin-12-p70 cytokine production was higher (P=0.04) in lymphocytes from controls with two alleles of the 5-bp insertion. IRF5 polymorphisms contribute to the risk profile for Crohn's disease and ulcerative colitis along with ancestry and NOD2 genotypes.