We report a case of sputum and dyspnea with patchy migratory air space infiltrates that developed in a 49-year old woman after she started breast radiation therapy following surgery for breast carcinoma. Our case clearly differed from ordinary radiation pneumonitis. Chest roentogenography and computed tomographic (CT) scanning demonstrated alveolar opacities. Bronchoalveolar lavage showed a moderate elevation of the total cell concentration and a considerable increase of lymphocytes, and transbronchial lung biopsies revealed a histological pattern of bronchiolitis obliterans organizing pneumonia with intra-alveolar granulation tissue. The present case suggests that breast irradiation may contribute to the development of a histological pattern of bronchiolitis obliterans organizing pneumonia.