The uncommon bronchiolitis obliterans organizing pneumonia can be idiopathic or caused by infection or medications. We report a 5 year old boy with pericardial sarcoma that was treated with chemotherapy (vincristine, doxorubicin, etoposide, cyclophosphamide) and radiotherapy. During the third cycle of chemotherapy, he developed progressive cough and dyspnea, needing oxygen therapy. Chest X-ray examination showed bilteral infiltrates. After discarding infectious etiologies, an open lung biopsy was performed, and the pathological examination showed a bronchiolitis obliterans organizing pneumonia. He was treated with prednisolone for 7 days, followed by prednisone for additional 45 days. He continued with fluticasone propionate as maintenance and remains in good condition with a ches X ray showing mild interstitial images, after seven months of follow up. (Rev Méd Chile 2000; 128: 633-40).