In five patients, rectal bleeding was ascribed to a patchy or diffuse mucosal colitis affecting the sigmoid colon only. The patients all had well marked localized sigmoid diverticular disease. The histologic features were nonspecific, without any changes suggestive of Crohn's disease. One patient had a short, self-limited illness, whereas the others had continuing illnesses lasting at least six months. In one patient who underwent sigmoid colectomy, the inflammatory features had disappeared, probably in response to sulphasalazine therapy. It is suggested that this syndrome may be a complication of the muscular disorder that underlies the formation of colonic diverticula.