The term "obstructive colitis" refers to ulceroinflammatory lesions occurring in the colon proximal to an obstructing or potentially obstructing lesion. We studied nine cases identified over a 9-month period. The patients were predominantly female (only one was male), elderly (mean age, 73), and usually had hypertension, diabetes, or other prior chronic illness. The colonic obstruction was due to adenocarcinoma in seven cases and to diverticular disease in two cases. Areas of colitis occurred either as circumscribed ulcers 0.5-2 cm in diameter (three cases) or as confluent circumferential lesions 8-25 cm in length; they were always separated from the more distal obstructing lesions by a segment of normal colon measuring 2.5-35 cm (mean, 14.6 cm). The involved area of colon was usually only mildly dilated; it exhibited moderate thickening of the wall and had a granular luminal surface accentuated in areas by deeper longitudinal or transverse ulcers. Often there were scattered pseudopolyps, and the margin separating the lesion from normal mucosa was well demarcated and irregular. In one case, two distinct separate areas of colitis were present; in another, the appendix was acutely inflamed. Microscopically, the lesions were composed of granulation tissue with a mixed acute and chronic inflammatory infiltrate that replaced the mucosa and often the submucosa; sometimes it extended into the muscularis propria, with associated peritonitis and perforation. Many of the features of obstructive colitis suggest an ischemic origin, probably mainly due to hypoperfusion following raised intramural pressure, but altered fecal flora may have a synergistic effect. The features of the disease are usually sufficiently characteristic to distinguish it from Crohn's disease and other forms of colitis. Complications include peritonitis, perforation and breakdown of anastomoses made through involved segments of colon that may appear externally normal at surgery.