Perfusion lung scanning was performed in 49 patients with lung cancer. The relationships among the percent of tumor-bearing lung perfusion, invasion of the lung cancer, and operability of the tumor were studied. The results showed that the reduction in perfusion was related to involvement of the hilum (t = 4.1, P less than 0.001), and that when the relative perfusion of the tumor-bearing lung was less than one third (33%) of the total lung perfusion, the lung cancer was most likely inoperable (chi 2 = 13.9, P less than 0.0002). In this paper, predictive postoperative FEV1.0 calculated by combining the result of spirometry with quantitative measurement of lung perfusion was compared with actual postoperative FEV1.0 in 19 (19/49) patients. FEV1.0 after pneumonectomy was accurately predicted by the modified Neuhaus' equation (r = 0.929 3, P less than 0.001). Postlobectomy FEV1.0 could be predicted effectively by direct quantitative equation (r = 0.850 2, P less than 0.001), Our results showed that perfusion lung scanning is a very valuable method for preoperative examination of lung cancer. It may enable patients with unresectable lung cancer to save the trouble of exploratory thoracotomy and those with poor pulmonary function to seize the opportunity of surgical treatment.