Crohn's disease (CD) and ulcerative colitis (UC) show an intestinal activation of T cells and macrophages within the inflamed lesions. The aim of the present prospective study was to determine whether circulating interleukins (IL) represent useful markers of immune activation in vivo and to characterize their respective roles in monitoring disease activity. Serum concentrations of the soluble IL-2 receptor (sIL-2R), IL-6 and IL-1 beta were measured in 10 patients with CD and 10 patients with UC before, at day 10 and 2 years after resection of inflamed bowel segments. The data were correlated with neopterin, C-reactive protein and other standard parameters of disease activity. Preoperatively, mean sIL-2R concentration was 495 +/- 62 U/ml (mean +/- SEM; healthy controls; 210 +/- 25 U/ml; p less than 0.02) in CD and 705 +/- 120 U/ml (p less than 0.00002) in UC. The corresponding IL-6 serum concentrations were 37 +/- 6 U/ml in CD (controls: 11 +/- 0.6 U/ml; p less than 0.0036) and 33 +/- 6 U/ml (p less than 0.04) in UC. Two years postoperatively, sIL-2R was still elevated in 6 out of 9 patients in both disease groups. These patients did not differ from the remaining group with respect to disease activity. Serum IL-6, elevated in 7 patients with CD and in 6 patients with UC at day 10 postoperatively, had returned to normal in all patients by this time.(ABSTRACT TRUNCATED AT 250 WORDS)