Fifty patients with Crohn's disease were studied endoscopically 6 weeks to 6 months (median 9 weeks) after ileocecal or ileocolonic resection for evidence of non-resected abnormality. Only 8 of the 50 patients were endoscopically free of abnormalities. Microscopic examination of the surgical specimen revealed inflammatory pathology at the site of resection line in 30 patients. Of the latter, 27 had endoscopic lesions at the anastomosis or in the "neoterminal" ileum. Of the 42 patients with endoscopic lesions only 8 had minor biochemical abnormalities. This study suggests that the well known reappearance of inflammatory activity after resection, especially at and proximal to the anastomosis, develops from foci of remaining inflammation and should be considered as recrudescence of the disease rather than true recurrence.