A 47-year-old man was referred to our institution with bloody diarrhea. An endoscopic examination showed a 6-cm tumor with central ulceration in the upper rectum. A high anterior resection of the rectum with lymphadenectomy was performed with the diagnosis of colorectal cancer. An intraoperative cytological examination found many free cancer cells in the peritoneal lavage fluid. Histologically, the tumor had invaded deeply and exposed the serosa, and was diagnosed as a small cell carcinoma of the colorectum with marginal lymph node metastasis. The patient is alive without any evidence of recurrence approximately 37 months after surgery despite the aggressive clinical behavior and a high mortality rate associated with this tumor. This case of an advanced small cell carcinoma of the colorectum showed a good outcome even though the cytology of the peritoneal lavage was positive.