We report a 64-years-old woman who underwent sparing mastectomy with adjuvant radiotherapy for breast cancer. One month after the end of radiotherapy, she presented with malaise, fever, fatigue, cough and migratory bilateral pulmonary infiltrates on serial radiological images. The microbiological studies of broncha alveolar lavage were negative. The patient under went a trans bronchial biopsy and the pathological diagnosis was compatible with an organizing pneumonia presumably associated with radiotherapy. Systemic steroid treatment was successful with rapid and complete resolution of clinical and radiographic manifestations.