[en] We report three cases of bronchiolitis obliterans organizing pneumonia (BOOP) that developed after irradiation therapy following breast cancer. All patients presented with cough and fever for 3 to 10 months after the completion of irradiation. Chest radiographs and computed tomography (CT) in all three patients demonstrated a consolidation outside the irradiated fields. Their laboratory data revealed increased C-reactive protein and increased erythrocyte sedimentation rates. Transbronchial lung biopsy was performed in all patients, and plugs of granulation tissue in the bronchioles and interstitial infiltration by mononuclear cells were found. Corticosteroid treatment resulted in rapid clinical improvement. BOOP was diagnosed from the histological and clinical findings. Although the etiology of BOOP still remains unknown, there may be a subgroup of such patients in whom the BOOP is induced by irradiation for breast cancer. These cases were assumed to be in a series of reported cases of BOOP primed by radiotherapy. (author)