The plasma concentration of human leucocyte elastase (HLE), measured by enzyme immunoassay as the complex with alpha 1-proteinase inhibitor, was determined in 94 patients with active and inactive inflammatory bowel disease. In Crohn's disease and in ulcerative colitis human leucocyte elastase levels were raised significantly above normal when the disease was active, and fell on remission. The mean human leucocyte elastase level in 31 cases of active Crohn's disease was significantly greater than the mean human leucocyte elastase level in 23 patients with active ulcerative colitis (p = 0.013). The values of human leucocyte elastase correlated significantly with Crohn's disease activity index scores (p = 0.05) and with the circulating concentration of C-reactive protein (p less than 0.05 and p less than 0.01 for ulcerative colitis and Crohn's disease respectively), but not with the erythrocyte sedimentation rate. These results indicate that the concentration of human leucocyte elastase in the plasma of patients with inflammatory bowel disease reflects the activity of their intestinal disease and suggest that serial measurements of human leucocyte elastase may be useful in the assessment and clinical management of these conditions.