Angio-, histo- and microangiographic studies were performed on five operative specimens of chronic regional enteritis affecting the ileum. In every patient the vascular pattern differed greatly from that seen in normal ileum and the changes were similar in each investigation. The number of long vasa recta and short vasa recta were increased, but the hypereaemic reaction was limited to the submucosa and mucosa. The long vasa recta arising from the marginal artery arcades were three to five times as close together in the affected regions as normally but their calibre was smaller than normal. The vessels were evenly spaced and ran without variation in their calibre up to the antimesenteric border of the gut. The thicker the mesentery, the wider the angle where the long vasa recta divided to form an anterior and a posterior branch. When the wrapping phenomen occurred this angle exceeded 90 degrees. The short vasa recta branching from the long vasa recta formed a very thin vascular network and thus the affected part of the gut could be easily distinguished from the normal ileum. The vascular changes were limited to the regions that were macroscopically abnormal. The arteries were well organized, but in the most severe cases where the architecture of the gut was destroyed the vascular pattern was also confused. The vascular pattern differed from that seen in malignant disease in two respects: there were no clear changes in the vascular calibres even in the most severe cases and no evidence of open arteriovenous shunts. The vascular changes were regarded as secondary to the fibrotic reaction in chronic regional enteritis and accurately reflected the severity of this reaction.