Amont 338 women treated in Massachusetts with isoniazid (isonicotinic acid hydrazide = INH) for pulmonary tuberculosis, no excess cancer deaths occurred (8 observed vs 8.3 expected) after 23 years (12.9 mean) of follow-up. There was an excess of cancer deaths (54 vs 35.7) among 1,090 patients who did not receive INH, partly due to radiogenic breast cancer resulting from multiple chest fluoroscopies to monitor pneumothorax. Increased deaths from liver cirrhosis (5 vs 0.8) were observed following INH use, suggesting that chronic as well as acute liver disease complicate this treatment.