The relationship between plasma interleukin-6 (IL-6) concentration and its soluble receptor in Crohn's disease (CD) is not well elucidated. Twenty healthy volunteers and 94 consecutive patients with CD (44 in relapse and 50 in remission) were studied. Plasma IL-6 concentrations in patients with active disease [80 +/- 9 pg/ml; mean +/- standard error of the mean (SEM)] were significantly higher than in patients with inactive disease (50 +/- 4 pg/ml; P < 0.001) or controls (3 +/- 1 pg/ml; P < 0.001). However, concentrations did not vary with the severity of CD attacks. Plasma concentrations of soluble interleukin-6 receptor (sIL-6R) in active-CD patients (77 +/- 5 ng/ml) did not differ significantly from those with inactive disease (82 +/- 5 ng/ml), while both groups had significantly raised concentrations compared with those of controls (58 +/- 6 ng/ml; P < 0.03 and P < 0.01, respectively). Plasma IL-6 concentrations correlated significantly with serum C-reactive protein (CRP) (r = 0.34; P < 0.001), whereas plasma sIL-6R concentrations did not. Taken together, these data suggest that, although IL6 and sIL6-R are both involved in the inflammatory process of CD, they are poor markers of disease activity.