Twelve patients undergoing pneumonectomy for tuberculous destroyed lung were studied to determine the value of preoperative spirometry in the assessment of their pulmonary reserve, and prediction of their postoperative morbidity. Preoperatively, the mean functional losses (as percentage of predicted values) were 44.5% of FVC, 54% of FEV, and 44% of PFR. No significant alteration occurred in FVC, FEV1 or PFR post-pneumonectomy. All patients were in Class 1 (NYHA) pre- and postoperatively. It is concluded that operability in these patients cannot be based on broncho-spirometry. The assessment of preoperative clinical (symptomatic) stage and the cardiovascular status appears to be the major determinants of pulmonary functional results.