Intraoperative small bowel endoscopy was performed on 33 occasions in 31 patients with Crohn's disease. The extent of mucosal inflammation was compared with that of changes in the external bowel wall: serositis, fat-wrapping and mural thickening. The influence of endoscopic findings on surgical management was evaluated. Mucosal inflammation was generally more extensive than serositis (P < 0.01), but less so than mural thickening (P < 0.001). The extent of fat-wrapping did not differ from that of mucositis. Of 23 patients undergoing reoperation or with fistula or abscess, however, eight had serositis and/or fat-wrapping in bowel segments without mucosal inflammation. Endoscopic findings influenced surgical decisions on 20 of the 33 occasions, limiting planned resection in 14, identifying strictures for repair in one, and deciding against resection in two cases and for extended resection in three. These results suggest that external inflammatory changes are unreliable guides to the extent of intestinal mucositis and requirements for resection in Crohn's disease. By visualizing the mucosa, intraoperative enteroscopy can provide information for more precise surgery, thereby limiting resection.