A 49-year-old female presented with productive cough, fever and chest pain, and was admitted to Nagoya University Hospital. Her chest X-rays, taken previously and on admission, showed infiltrative shadows in both upper lung fields and left-sided pleural effusion. Rheumatoid factors were positive in serum and the pleural effusion. Antibiotics were ineffective. Transbronchial lung biopsy revealed intraalveolar fibrosis as well as interstitial inflammation. Idiopathic BOOP was suspected on the basis of clinical findings together with the histological features. However, open lung biopsy revealed lymphoid hyperplasia with germinal center formation. The patient was diagnosed as having lung involvement antedating rheumatoid arthritis, despite the absence of joint symptoms at present.