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Interleukin-18 (IL-18) is a pleiotropic cytokine that induces the production of interferon (IFN)-γ and also to regulate Th2 cytokines. Recently, association studies between IL-18 gene promoter polymorphisms and several Th1- or Th2-mediated inflammatory diseases were reported. In inflammatory bowel diseases (IBD), including ulcerative colitis (UC) and Crohn's disease (CD), recent evidence suggests that IL-18 is involved in the pathogenesis. Methods: Using DNA direct sequencing, we investigated IL-18 gene promoter polymorphisms at −607C/A and −137G/C. Allele, genotype, and haplotype frequencies were determined in 210 Japanese patients with UC, 205 patients with CD, and 212 controls. Results: In UC, the −137C allele frequency was significantly higher in the proctitis-type patients than in controls (Pc = 0.0068). The −137 genotype frequency was also significantly different in the proctitis-type patients than in controls (Pc = 0.032). No other allele and genotype frequencies were significantly associated with UC after Bonferroni correction. Furthermore, the frequency of haplotype 2 (−607A, −137C), which had a lower promoter activity and IFN-γ mRNA level than the other haplotypes as previously reported, was significantly higher in the proctitis-type patients than in controls (Pc = 0.01). In CD, we could not find any significant differences. Conclusions: IL-18 gene promoter polymorphisms may not be associated with disease susceptibility but related to the extent of disease in UC.