A 38 year old female with treated carcinoma of the breast presented with a clinical and radiological diagnosis of bronchiolitis obliterans organising pneumonia. Based on these findings a biopsy was not performed and she was treated with steroids. Following lack of response to steroid therapy a lung biopsy was performed showing peribronchiolar, lymphatic, venular and arteriolar metastatic carcinoma. Her symptoms responded to chemotherapy. Interstitial and intracellular spread of carcinoma should be considered in the differential diagnosis of the radiological appearance of BOOP, and therefore a histological diagnosis should be obtained to determine appropriate management.