We report here a case of Pancoast's syndrome that developed 15 years after a radical mastectomy for right breast cancer. The patient had undergone a radical mastectomy on February 1, 1979. Following surgery, numbness and pain persisted from the right shoulder to the right anterior chest region, and in 1993, the pain gradually became stronger. Right blepharoptosis occurred in 1994, and lymph node swelling in the supraclavicular region was noted in June of the same year. Aspiration biopsy was performed on the lymph nodes, and a cytological diagnosis of metastatic adenocarcinoma was made. Since the findings from a CT scan suggested Pancoast-type cancer, we performed a right upper lobectomy and dissection of the mediastinal lymph nodes, including the right supraclavicular lymph nodes. However, pulmonary and lymph node metastases were diagnosed 15 years after the breast cancer operation. The patient underwent chemo-endocrine therapy and irradiation after the operation and remains well at the present time, five years after this operation.