Two groups of patients were compared who were submitted to thoracotomy for carcinoma of the bronchus and had their respiratory function tested prior to operation. One group consisted of all the patients tested over a 6½-year period who suffered post-operative cardiopulmonary complications.The complications occurred more often in the patients with operations on the right lung than in the others. When the findings were differently presented it appeared that the patients who had more than half the total amount of lung tissue removed (i.e. a right pneumonectomy) had a higher incidence of complications than the rest.There were significant differences between the respiratory function test results (other than the vital capacity values) of the patients with complications and those without.There were no differences in the test results or in the nature of the operations between the patients who survived the complications and those that died.It was concluded that help might be obtained in assessing the possible risk of the complications following thoracotomy for carcinoma of the bronchus (but not of death from those complications) from the pre-operative respiratory function test results and whether or not the anticipated operation is a right pneumonectomy