At present surgery is accepted as the most effective mode of therapy for carcinoma of the lung. Because the lack of respiratory reserve is the major determinant of postoperative function, it is useful to identify the patient, who is at significant risk. Eighteen patients with lung cancer (mean age = 56 +/- 6.5 years) were studied preoperatively (preop) and postoperative (postop) (three to four months after lung resection) by spirometry, measurement of arterial blood gases, and quantitative lung scanning (99mTc). A predicted postoperative value of some variables was calculated by the formula: postop value = preop value x % function of regions of lung not resected. The correlation coefficient between the predicted (pred) and postoperatively observed (observ) values VC = vital capacity, FEV1 = forced expiratory volume in 1 second) is: VC pred/VC observ r = 0.83 p less than 0.001 FEV1 pred/FEV1 observ r = 0.82 p less than 0.001. The authors' results agree with earlier reports and show that the method used can accurately predict the postoperative respiratory function in patients undergoing lung resection (pneumonectomy or lobectomy). A predicted FEV1 of 0.8 L does not permit a surgical program, because, below this level, carbon dioxide retention becomes more frequent and exercise intolerance is increasingly severe (poor quality of life). The method proposed to predict the postoperative respiratory function is simple and routinely useful. The authors choose a perfusion instead of ventilation scan, because the former provides similar predicted postoperative data, and can be done routinely.