We describe a patient with rheumatoid arthritis(RA) who developed bronchiolitis obliterans organizing pneumonia(BOOP) during the treatment of bucillamine. A 51 year-old man was admitted to the hospital for an abnormal shadow on his chest radiograph. He had been diagnosed as having RA 3 years previously and had been receiving 200 mg of bucillamine for 21 months. Two months prior to admission, he presented with a cough and his chest X-ray showed opacities in both lower lungs. He was treated with antibiotics for 2 months after the development of cough and lesions on the chest X-ray, but the symptoms and lung lesions became more aggravated. On admission, an HRCT revealed airspace consolidations in the subpleural space of both basal lungs and a CT-guided fine needle aspiration biopsy showed Masson's body filling air space, interstitial infiltration of acute and chronic inflammatory cells and type II cell hyperplasia, consistent with BOOP. Bucillamine was stopped and 50 mg of prednisolone was administered. His symptoms and infiltrations on the chest X-ray resolved. We suggest that bucillamine should be considered as a drug possibly associated with BOOP.