In a prospective study we compared the usefulness of various laboratory tests (albumin, alpha-1-proteinase inhibitor (A1PI), cholinesterase (CHE), C-reactive protein, erythrocyte sedimentation rate, hematocrit) and activity indices (CDAI, VHAI) in relation to the disease activity by endoscopic criteria. Except for hematocrit highly significant differences (p less than 0.0005) of the mean values of all test results were found for patients without or with slight mucosal lesions compared with patients with severe inflammation of the mucosa. Further analysis of the data indicates the highest test efficiency (84%), sensitivity (80%), and specificity (88.6%) for CHE. CHE showed good correlations to all other tests; the highest correlation was found between CHE and VHAI (r = -0.78). We suggest that a suppression of CHE synthesis mediated by endotoxins and cytokines rather than an increased intestinal loss explains the decreased CHE in severe Crohn's disease. It is concluded from the data that CHE is a useful test to assess the inflammatory activity of Crohn's disease.