A case of obstructive colitis associated with rectal carcinoma in a 56 year old Japanese man is reported herein. He presented to Shinkokura Hospital with severe abdominal pain following a one month history of anal bleeding and mild abdominal pain. On palpation, muscle guarding was observed in the left lower quadrant and the white blood cell count was 14,200/mm3. An exploratory laparotomy was performed under the provisional diagnosis of acute abdomen, which revealed localized peritonitis 8 cm oral to an area of rectal carcinoma. An anterior resection of the lesion was therefore performed together with a descendo-proctostomy. The histopathologic diagnosis revealed adenocarcinoma and obstructive colitis involving the entire thickness of the sigmoid colon and resultant fibrino-purulent peritonitis. His post-operative course was uneventful and he was continuing to do well on the 30th postoperative day, at the time of writing. The clinical significance of this combination of obstructive colitis with rectal carcinoma is briefly discussed following the presentation of this case.