Patients with metastatic breast carcinoma often present symptomatic pleural effusions. A 67-years-old woman, who had undergone right radical mastectomy 7 years previously for right breast carcinoma, developed massive right pleural effusion. Thoracoscopy revealed local metastasis in the center of the parietal mediastinal pleura. On biopsy, it was found to be adenocarcinoma which was considered to have invaded the pleura directly from the mediastinum. She was treated with linac X-ray irradiation focused on the pleural metastasis and systemic chemotherapy (CMF), and resulted in a gradual decrease and final disappearance of the pleural effusion.