We report a case of organizing pneumonia (OP) that developed after radiation therapy (RT) for breast cancer. A 54-year-old woman presented with malaise and fever within a month after the completion of RT for breast cancer. Chest radiographs and computed tomography (CT) demonstrated consolidation in the left upper lobe consistent with radiation pneumonia. The patient was given 60 mg/day IV cortisone for 15 days after which her complaints and consolidation in the left upper lobe disappeared. The daily dose of her corticosteroid was tapered down to 20 mg/ day. Two weeks later, the patient again had fever and malaise. Chest X-ray and CT revealed bilateral pulmonary opacities located outside the irradiated fields, predominantly in the middle and lower lung zones. The patient's laboratory tests were normal except for her erythrocyte sedimentation rate, which was elevated. Bronchial lavage revealed moderate elevation of the total cell number with lymphocyte predominance. Open lung biopsy was performed and histopathological examination demonstrated findings consistent with OP. High dose (60 mg/day) prednisolone treatment resulted in rapid clinical and radiological improvement. When the prednisolone dose was gradually tapered down to 20 mg/day during follow-up, new pulmonary opacities developed in both lungs, as well as the recurrence of the patient's symptoms. Increased dose of prednisolone resulted in the rapid improvement of the clinical symptoms and radiological abnormalities. OP rarely presents after RT for breast and lung cancer. One should always consider OP in the clinical setting of a patient who has a history of RT completed 3-6 months prior to fever, multiple areas of consolidation, and ground glass opacities outside the RT field.