The activity of diamine oxidase (DAO), an enzyme found in the apical villous cells of the small intestine mucosa, should reflect the status of the intestinal mucosa. Our purpose was to determine whether DAO activity in the intestinal mucosa is diminished in patients with Crohn's disease and correlates with the severity of histological changes. Mucosal DAO activity was determined in 42 tissue specimens from patients with Crohn's enteritis (n = 15), Crohn's colitis (n = 9), and ulcerative colitis (n = 11), and from patients with no intestinal disease (n = 7). DAO activity was estimated by the metabolism of [14C]putrescine. Histologic changes were graded on a scale of 0-4. Normal histology was graded as zero, mild edema, and inflammation in the lamina propria as one, crypt abscess formation and inflammation as two, more severe inflammation plus or minus granulomata as three, and most severe inflammation with active ulceration as four. Tissue DAO activity was significantly less in patients with ileitis (4.8 +/- 3.6) compared with those with Crohn's colitis (15.0 +/- 11.6), ulcerative colitis (16.8 +/- 19.7), and normal intestine (17.6 +/- 14.3 U/mg protein/h, p less than 0.05). Intestinal DAO activity showed a positive correlation with the histologic scores. Recurrence of Crohn's disease developed postoperatively in two patients with low tissue DAO activity (1.4 and 2.9 U/mg protein/h). Intestinal DAO activity is diminished in patients with Crohn's ileitis and correlates with the severity of histologic changes. Tissue DAO activity might prove useful in predicting the risk of recurrence or anastomotic complications after resection for Crohn's disease.