If the postoperative pulmonary function can be predicted in the patients undergoing pneumonectomy for lung cancer preoperatively, it will be helpful for identifying them as high or low risk candidates. Perfusion lung scan and spirometry were performed in 12 patients with lung cancer pre-operatively and the predicted postoperative Vital Capacity, FVC, FEV1.0, FEF25 - 75% and MVV were estimated by multiplying the preoperative values by the percentage of perfusion of the nonsurgical lung. Three months postoperation the patients were reinvestigated with spirometry and the obtained values were compared with the predicted values. The linear regression lines derived from the correlation between predicted values [X] and observed values [Y] were as follows; VC; R=0.532, Y=0.48X+1.28, P=0.075 FVC; R=0.566, Y=O 54X+1.15, P=0.055 FEV1.0; R=0.832, Y=0.68X+0.70, P=0.001 FEF25 ~ 75%; R=0.781, Y=0.68X+0.54, P=0.003 MVV; R=0.718, Y=0.67X+34.75, P=0.009 The prediction of postoperative FEV1.0, FEF25 ~ 75% and MVV in lung cancer patients undergoing pneumonectomy appear to be valid for evaluating preoperative pulmonary function.