Interleukin-18 (IL-18) is believed to be one of the most important cytokines in the pathogenesis of inflammatory bowel disease (IBD). The aim of the study was to clarify the significance of single-nucleotide polymorphisms (SNPs) at the 5′-end of the IL-18 gene in the development of IBD. DNA was obtained from peripheral blood of 99 patients with ulcerative colitis (UC), 79 patients with Crohn's disease (CD), and 102 healthy controls. All participants were Japanese. SNPs at -656G/T, -607C/A, -137G/C, +113T/G, and +127C/T were determined by means of direct sequencing, and a genetic association with IBD was examined. The frequencies of the G allele at +113 and the T allele at +127 were significantly higher in patients with CD and UC compared with controls. The differences in allelic frequencies were more striking in patients with CD than in patients with UC, and at position +127 than at position +113. The haplotype estimation, according to the E-M algorithm, suggested that TACGT is closely associated with IBD, especially with CD. It was concluded that SNPs at the 5′-end of IL-18 gene might be closely related to the etiology of IBD.