In a number of patients, radiotherapy following surgery for breast carcinoma may induce radiation injury to the lungs. This has classically been divided into an early radiation pneumonitis and a late fibrosis, both confined to the irradiated lung volume. However we observed a female patient who similarly to other recent reports in the literature developed a recurring pneumonitis migrating from one lung to the other after radiotherapy for breast carcinoma. This migratory BOOP (bronchiolitis obliterans organizing pneumonia) was characterized by a lymphocytic alveolitis and responded well to corticosteroids. Clinicians should be aware of the possibility of a lymphocytic pneumonitis in both lungs after unilateral thoracic irradiation and recognize the distinctive features of fever, cough, dyspnoea and malaise in order to start an effective treatment with corticosteroids. They should also be aware of the high tendency for recurrence when tapering off.