Vascular changes were seen in specimens with lesions of Crohn's disease as well as in ulcerative colitis. In Crohn's disease, the vasculature in the bowel wall was rearranged secondary to thickening of the intestinal coats. An anatomical relationship was often found between the ulcers and the point where arteries break through the muscularis propria. The vasculature was markedly increased in areas with deep ulcers and transmural inflammation, but in areas with only small ulcers and mild inflammatory reaction vascular changes were barely discernible or absent. Occlusive vascular changes were uncommon and not visible in the radiographs. In active ulcerative colitis hypervascularity was more marked than in Crohn's disease, but was confined to the mucosa and adjacent parts of the submucosa. In ;burnt-out' atrophy vascularity was not increased and only minor changes in the submucosal arteries were observed. The observations did not suggest that the vascular alterations are of primary importance for the formation of lesions in either disease.