60 patients have been entered into a randomised prospective study to answer the question-does a single postoperative injection of B.C.G. into the pleural space improve survival after surgery for lung cancer? 40 patients have been followed up for more than a year. B.C.G. improved survival in patients with a limited tumour burden: there were no recurrences and no deaths in 17 stage-I patients treated with intrapleural B.C.G. whereas 9 of 22 comparable control patients developed recurrent cancer and 5 died in the same interval (p=0.003). Intrapleural B.C.G. treatment did not seem to be beneficial in patients with more advanced disease. The hazards associated with injecting these living organisms into the pleural space have been reduced by preclinical laboratory testing in animals, use of a single limited dose of microorganisms, administration of isoniazid, and careful patient monitoring.