Nuclear Factor-κB (NF-κB) is a major transcription regulator of immune response, apoptosis and cell-growth control genes, and is upregulated in inflammatory bowel disease (IBD), both ulcerative colitis (UC) and Crohn's disease. The NFKB1 gene encodes the NF-κB p105/p50 isoforms. Genome-wide screens in IBD families show evidence for linkage on chromosome 4q where NFKB1 maps. We sequenced the NFKB1 promoter, exon 1 and all coding exons in 10 IBD probands and two controls, and identified six nucleotide variants, including a common insertion/deletion promoter polymorphism (−94ins/delATTG). Using pedigree-based transmission disequilibrium tests, we observed modest evidence for linkage disequilibrium (LD), independent of linkage, between the −94delATTG allele and UC in 131 out of 235 IBD pedigrees with UC offspring (P=0.047–0.052). This allele was also more frequent in the 156 non-Jewish UC probands from the 235 IBD pedigrees than in 149 non-Jewish controls (P=0.015). The −94delATTG association with UC was replicated in a second set of 258 unrelated, non-Jewish UC cases and 653 new, non-Jewish controls (P=0.021). Nuclear proteins from normal human colon tissue and colonic cell lines, but not ileal tissue, showed significant binding to −94insATTG but not to −94delATTG containing oligonucleotides. NFKB1 promoter/exon 1 luciferase reporter plasmid constructs containing the −94delATTG allele and transfected into either HeLa or HT-29 cell lines showed less promoter activity than comparable constructs containing the −94insATTG allele. Therefore, we have identified the first potentially functional polymorphism of NFKB1 and demonstrated its genetic association with a common human disease, ulcerative colitis.