The significance of interleukin-6 (IL-6) in patients with inflammatory bowel disease (IBD) was studied by measuring the IL-6 level in serum and colonic tissue by means of an enzyme-linked immunosorbent assay (ELISA), and by examining its localization using an immunohistochemical method. The serum IL-6 level reflected the degree of disease activity, and the extent of affected area, and was also correlated with the serum C-reactive protein (CRP) level. In the colonic mucosa of active IBD, the tissue IL-6 level was markedly elevated, and immunoreactive products of anti-IL-6 antibody were present in infiltrative mononuclear cells in the lamina propria. This indicates that IL-6 production in these cells is enhanced at the site of affected intestine. These results, together with its biological activity and the type of cell producing it, suggest that IL-6 is an available marker to assess disease conditions of IBD and that it might be also involved in the pathophysiology of IBD.