Interleukin-6 (IL-6) has a major function in the regulation of the inflammatory process. We aimed to define its role as a parameter of disease activity and extent in inflammatory bowel disease. Serum concentrations of IL-6 were measured in 28 patients with Crohn's disease (CD) and in 15 with ulcerative colitis (UC) before starting corticosteroid treatment. Disease activity was measured by standard activity indexes. Serum IL-6 levels were increased in patients with CD (36 +/- 8 pg/ml; p < 0.001) and UC (10 +/- 4 pg/ml; p < 0.05) as compared with 25 control patients. A significant correlation between serum IL-6 concentrations and disease activity was found in patients with CD as well as in patients with UC (active CD: 73 +/- 14 pg/ml, inactive disease: < 10 pg/ml, p = 0.003; active UC: 26 +/- 10 pg/ml, inactive disease: < 10 pg/ml, p = 0.004). IL-6 serum levels were related to the acute-phase reactant c-reactive protein (r = 0.51, p < 0.01) in CD patients. The serum IL-6 concentrations were more pronounced in CD of the colon than in disease limited to the small bowel (p < 0.05). In patients with CD as well as in patients with UC, IL-6 serum concentrations showed a higher sensitivity for disease activity (94 and 83%) than serum c-reactive protein levels. In patients without corticosteroid treatment, the IL-6 serum concentration is related to disease activity in CD as well as UC. Serum IL-6 levels show a higher correlation with disease activity than c-reactive protein levels.