Abstract We predicted the postoperative forced expiratory volume in 1 second (FEV 1) with a formula based on the premise that the total number of subsegments was 42: postop FEV 1 = [1 – (b – n)/(42 – n)] (preop FEV 1), where n and b are the number of obstructed subsegments and total subsegments, respectively, in the resected lobe. It was assumed that b was 6, 4, and 12 in the right upper, middle, and lower lobes, respectively, and 10 each in the left upper and the left lower lobes. The obstructed subsegments, n, were obtained from the findings on bronchography or bronchofiberscopy or both before operation. The linear regression line derived from the correlation between predicted (x) and measured ( y) FEV 1 was y = 0.850 x + 0.286 ± 0.296 (standard error) (N = 52; r = 0.821; p < 0.001). We calculated the predicted postoperative FEV 1 in 188 patients with primary lung cancer. The predicted values were corrected with the regression equation just mentioned and then normalized by the patient's height and sex (%FEV 1(p,c)). The correlation between %FEV 1(p,c) and the surgical risk was studied. Postoperative respiratory complications were inversely related to %FEV 1(p,c), and a significantly high incidence of complications ( p < 0.05) was observed in those whose %FEV 1(p,c) was less than 60% of predicted normal. In aged patients (65 years old or more) without complications, %FEV 1(p,c) was 67.3 ± 18.0%; it was 52.2 ± 12.8% in those with respiratory trouble and 53.3% ± 9.6% in those with circulatory complications. The difference between groups with and without complications was significant ( p < 0.01). In the nonaged group, there were no significant differences among the patients with no complications, respiratory complications, or circulatory complications. We conclude that our formula involving subsegments is a reliable method for predicting postoperative lung function and is a predictor of the surgical risk in aged patients with lung cancer.