Breast-conserving surgery and postoperative radiotherapy play an important role in the treatment of early breast cancer. Bronchiolitis obliterans with organizing pneumonia (BOOP) is an uncommon syndrome reported to be one of the complications of adjuvant radiotherapy. We report the case of a 71-year-old woman who developed cough, dyspnea and fever three weeks after radiation therapy to the left breast for breast carcinoma. Chest X-ray and computed tomography scan demonstrated alveolar opacities within both lungs. Antibiotic therapy against any probable septic pathology did not improve the symptoms, while corticosteroid treatment resulted in rapid clinical improvement together with regression of the pulmonary opacities. Irradiation was thought to be the cause of the migratory pneumonitis, hence this case was clinically diagnosed as radiation-induced migratory pneumonitis similar to BOOP, without lung biopsy. The present case suggests that one should be mindful of this disease when treating patients with a history of irradiation to the breast. BOOP promptly responds to systemic corticosteroid therapy with rapid improvement of symptoms and regression of the pulmonary opacities.