A case of isoniazid (INH)-induced lupus occurring in a 62-year-old man is presented. He visited our hospital in May 1986 and a cavitary lesion was found in the right upper lobe on a chest roentgenogram. He had no previous history of treatment with antituberculotic agents. Though acid-fast bacilli were not found in his sputum, pulmonary tuberculosis was strongly suspected and INH, rifampicin and ethanbutol were administered. Four days after starting the treatment, minimal left pleural effusion was seen on chest X-ray film. Three months later he began to complain polyarthralgia in his digital joints. In a pleural effusion many lymphocytes were found; and the antinuclear antibody (ANA), the antiextractable nuclear antigens (ENA) antibody, and the RNase resistant anti-ENA antibody were positive, and their titres were 20×, 1000× and 1000×, respectively, and the immune complex (IC) was 16.0μg/ml (LT 5). In blood serum, the ANA test the and-ENA antibody and the RNase resistant anti-ENA antibody were positive with titres 40×, 640× and 640× respectively;and the IC was 14.0μg/ml, and the RA test was positive. The improvement of clinical findings and disappearance of auto-antibodies seen after stopping INH confirmed the diagnosis as INH-induced lupus.