Polypectomy was conducted under rectoscopic observation for a patient with a small rectal polypoid cancer (mucosal cancer) on an outpatient treatment basis. The serum carcinoembryonic antigen (CEA) level, which had been high before the polypectomy, was reduced and complete cure seemed to have been attained. However, the postoperative serum CEA level at the patient's periodical check-up proved to be abnormally high, and a closer examination revealed a small pulmonary metastasis. Eight months after the polypectomy, the metastatic focus in the lung was resected curatively and the serum CEA level returned to normal. It was verified by immunofluorescence that the tissues in the metastatic focus were CEA-productive.As the pulmonary metastasis, which is extremely rare in the cases of early cancers, could be detected at its early stage by the periodical check of the serum CEA level, and as the serum CEA level could be returned to normal by resection of the metastatic focus, it was thought that the resection was curative.