The presence of a solitary lung tumor after radical surgery for breast cancer often causes difficulty in the diagnosis and treatment. We report a patient who had previously undergone radical surgery for breast cancer, and who underwent lung surgery. When the patient was 57 years old, she underwent surgical treatment for breast cancer. Six years later, a chest radiograph revealed a solitary mass on the upper lobe of her left lung. A left upper lobectomy was performed, utilizing videoassisted thoracic surgery. The pathological diagnosis, based on hematoxylin and eosin staining, was lung metastasis from breast cancer. The immunohistological findings revealed that the tumor was positive for estrogen receptor, progesterone receptor, and GCDFP-15, and negative for TTF-1 and Napsin A. Although the original breast cancer lesion was positive for the HER2 receptor, the metastatic lung lesion was negative. It has already been reported that the subtypes can change from 1 subtype in the original lesion to another in the metastatic lesion. Adjuvant therapy was administered based on the subtype of the metastatic lesion after the surgery. Therefore, surgical resection is useful for the purpose of reevaluating HER2 receptor status, the results ofwhich can influence the postoperative adjuvant treatment. The patient is now doing well, without any evidence ofrecurrence or metastasis 15 months after surgery.