Plasma postheparin diamine oxidase (DAO) activity has been evaluated for assessing disease activity in Crohn's disease (CD) and other intestinal disorders. Since the mechanism of the reduced plasma DAO activity is poorly understood, our aim was to determine the effect of extent and location of disease and prior resection and therapy on plasma DAO activity in Crohn's disease. Plasma postheparin DAO activity was significantly lower (17.4 +/- 3.0 vs 32.8 +/- 30.8 units/ml) and Crohn's disease activity index (178 +/- 105 vs 14 +/- 19, P less than 0.05) (CDAI) higher in 37 patients with CD compared to 30 normal volunteers. There was no overall correlation between DAO activity and CDAI. Effective medical or surgical therapy increased DAO activity and decreased CDAI, while clinical recurrence had the opposite effect. DAO activity was not related to the extent of small bowel disease (13.2 +/- 9.1; less than 30 cm, 18.5 +/- 11.8; 30-60 cm, and 5.7 +/- 6.4 units/ml; greater than 60 cm) or colonic disease (13.0 +/- 6.9 segmental vs 24.0 +/- 15.4 units/ml, pancolitis). DAO activity was similar with small or large bowel disease (14.3 +/- 10.6 vs 18.8 +/- 13.1 units/ml). Prior enterectomy or colectomy did not significantly influence DAO activity. DAO activity responds predictably after effective therapy and recurrence and may prove useful in monitoring individual patients with CD. Failure of extent and location of disease and prior resection to influence DAO activity suggests that DAO activity is not directly related to enterocyte mass.