Small cell carcinoma associated with ulcerative colitis is a rare malignancy of the colon, so we report here on a case of small cell carcinoma (SCC) of the large bowel. A 60-yr-old woman had been diagnosed with ulcerative colitis about 10 years previously, and she was then continuously treated with 5-ASA. Colonoscopy was being done every two years for cancer surveillance. The last colonoscopy was done 16 months previously. At that time, colonoscopic finding was mild left side colitis. She was hospitalized because of severe abdominal pain and acute constipation that developed 2 weeks previously. During the colonoscopic examination, an obstructive mass was noted on the rectum. Pathology revealed small cell carcinoma. NSE (neuron specific enolase) and Synaptohpysin were strongly positive. Chromogranin and Cytokeratin were focally positive to a weak degree. So, the patients received a Hartmann's operation (T-colostomy). The mass had grossly invaded the adjacent peritoneum and serosa. Therefore, only the mass was removed. She then received chemotherapy with cisplatin and irinotecan for 2 cycles. She achieved a stable state 2 months after the operation. Only a few cases of colorectal SCC associated with ulcerative colitis have been reported, and its etiology remains unknown. Thus, we report here on a small cell carcinoma arising from ulcerative colitis.