BACKGROUND & AIMS: Postoperative recurrence of Crohn's disease may be triggered by agents in the fecal stream. The aim of this study was to examine intestinal mucosal inflammation induced by contact with intestinal fluids in surgically excluded ileum. METHODS: The effects of infusion of intestinal luminal contents into excluded ileum in 3 patients with Crohn's disease who had undergone a curative ileocolonic resection with ileocolonic anastomosis and temporary protective proximal loop ileostomy were studied by histopathology and electron microscopy. RESULTS: Contact with intestinal fluids for 8 days induced focal infiltration of mononuclear cells, eosinophils, and polymorphonuclear cells in the lamina propria, small vessels, and epithelium in the excluded neoterminal ileum that was previously normal. Epithelial HLA-DR expression increased, and mononuclear cells expressed the KP-1 antigen associated with activation. Marked up-regulation of RFD-7, RFD-9, intercellular adhesion molecule 1, and lymphocyte function-associated antigen 1 was observed after infusion, reflecting epithelioid transformation and transendothelial lymphocyte recruitment. At the ultrastructural level, dilatation of the endoplasmic reticulum and Golgi apparatus occurred in epithelial cells, where also basally located transport vesicles were identified. CONCLUSIONS: Intestinal contents trigger postoperative recurrence of Crohn's disease in the terminal ileum proximal to the ileocolonic anastomosis in the first days after surgery.