Respiratory function test results from 243 selected patients with carcinoma of the bronchus were studied. One third of the patients eventually had post- thoracotomy cardiopulmonary complications, another third were complication-free, and the remainder were rejected as unsuitable for surgery.Limits were evolved to apply to combinations of test results which would separate patients needing similar operations in the future into four categories of risk of post-operative complications. This ategorisation could be taken in conjunction with clinical and other findings in arriving at a surgical decision, but could not be used on its own to determine whether or not a patient would be fit for operation