The key-enzyme for the metabolism of diamines in man is diamine oxidase (DAO). Its highest activities are in the intestinal mucosa, localized in the cytoplasm of the mature enterocytes of the small and large bowel. If the gut is affected by inflammation in Crohn's disease macroscopical changes are observed. This prospective study investigated if these mucosal alterations are also reflected in changes of mucosal diamine oxidase activity and/or mucosal histamine content respectively. Twenty patients (12 female, 8 male; age: means = 31, range 18-49 years) undergoing gut resection because of complications in Crohn's disease (Jan.-Dec. 1988) formed the basis of the study. Tissue samples of the resected material from areas inflamed and histologically not involved in the disease were investigated for diamine oxidase activities and histamine content. Diamine oxidase activities in the mucosa obtained from the macroscopically normal proximal (155.6; (76-393) mU/g (means, range)) and distal (132; (58.5-295) mU/g) resection margins were similar to our previous findings. In all patients, however, samples from the diseased mucosa had significantly (ca. 50%) lower diamine oxidase activities (74.5; (5-262) mU/g) compared to the healthy tissue. Similar differences were found in material obtained either from whole intestinal wall or from the mucosa. The determination of diamine oxidase activity constitutes possibly a more unambiguous and earlier parameter for assessing the extent of the inflamed area than histological disease presentations. Using biopsies the necessary extent of resection could be estimated before operation: this may influence operative strategies and help in the definition of the minimum amount of inflamed gut to be removed.