PURPOSE:Numerous investigators have shown increased bowel permeability in patients with Crohn's disease. It is unclear whether this is a precondition affecting the entire intestine or a consequence of the inflammation and, therefore, only affecting the diseased bowel. The present study tested the hypothesis that resection of diseased bowel in patients with ileocolonic Crohn's disease would correct abnormalities in bowel permeability.METHODS:Ten patients (5 females; mean age, 33±2 years) with ileocolonic Crohn's disease who underwent elective ileocolic resections had bowel permeability measured preoperatively and postoperatively by the relative urinary clearance of orally consumed lactulose and rhamnose.RESULTS:Mean preoperative bowel permeability in patients with Crohn's disease was significantly elevated relative to healthy volunteers (0.172±0.04vs.0.046±0.01;P<0.05, unpairedt‐test). After ileocolectomy, bowel permeability decreased in patients with Crohn's disease and reached a normal range by postoperative day 30.CONCLUSIONS:Bowel permeability is increased in patients with ileocolic Crohn's disease because of the presence of diseased bowel and decreases to a normal range in these patients after resection of gross intestinal disease. This suggests that bowel permeability may be a quantitative and clinically effective method with which to assess the presence and severity of diseased bowel in patients with Crohn's disease.