Plasma levels of interleukin-1 receptor antagonist (IL-1ra), a potent inhibitor of IL-1, were measured in patients with inflammatory bowel disease. Plasma IL-1 ra levels in patients with active ulcerative colitis and Crohn's disease were higher than in normal controls. No significant difference was noted in plasma IL-1 ra concentrations between active ulcerative colitis and Crohn's disease patients. The levels in patients with inactive disease were lower than in active patients, but were higher than in normal controls. Plasma IL-1ra levels correlated significantly with clinical disease activity and laboratory parameters such as C-reactive protein or leukocyte count. In conclusion, circulating IL-1ra in patients with inflammatory bowel disease may be a useful marker of disease activity.