We report herein the case of a 42-year-old woman in whom a solitary lung metastasis from stromal sarcoma of the breast was effectively treated by radiotherapy. The original breast tumor had been extirpated in a local hospital, and she was subsequently referred to our outpatient clinic for nonsurgical treatment. Pathological examination of the breast tumor had confirmed a diagnosis of stromal sarcoma. Radiation and systemic chemotherapy for the remnant disease were administered at our hospital; however, 8 months later, local recurrence and a solitary lung metastasis were recognized on a chest X-ray. A second lumpectomy of the recurrent breast tumor along with radiation for the lung metastasis and systemic chemotherapy were performed. After a 4-year disease-free period, there is still no sign of recurrence. This case serves to demonstrate the effectiveness of radiotherapy for the treatment of lung metastasis from stromal sarcoma of the breast.