To elucidate the possible role of proinflammatory cytokines in inflammatory bowel disease, the expression and localization of interleukin (IL) -6 and IL-8 mRNAs were examined in colonic biopsy specimens obtained from 10 patients with active ulcerative colitis (UC), 5 with inactive UC, 6 with Crohn's disease (CD), and 5 normal controls. In situ hybridization with digoxigenin-labeled probes and immunohistochemistry for both cytokines were performed. The IL-6 mRNA expression was enhanced in the inflamed mucosa in 4 of 6 CD patients, while that of UC patients stayed at baseline. In contrast, IL-8 mRNA expression was apparently augmented (P = 0.044) in 7 of 10 active UC and 3 of 6 CD patients (NS). The cell count positive for IL-8 mRNA per unit area was definitely increased in moderate/severe UC when compared to mild UC (53.1 +/- 14.4/mm2 vs 9.0 +/- 5.1/mm2, P = 0.028) according to the degree of inflammation. IL-6 mRNA positive cells in CD were preferentially located in deeper lamina propria than IL-8 mRNA positive cells in UC. Interestingly, IL-8 mRNA was expressed in the mucosal epithelial cells in one UC patient. The patients treated by corticosteroids tended to show suppressed expression of each mRNA, except one patient with intractable UC. Our data suggest enhanced expression of mucosal IL-6 mRNA in CD and of IL-8 mRNA in UC by infiltrating mononuclear cells, indicating the distinct participation of each cytokine in the pathogenesis of UC and CD. Moreover, intestinal epithelial cells in UC occasionally exhibit IL-8 mRNA.