Eighteen patients operated on for pulmonary cancer, the procedure varying from the removal of two segments to pneumonectomy, were subjected to measurement of the spirometric values VC, FEV1, FRC and RV preoperatively and 2-3 months postoperatively. The possibility of predicting the postoperative values from the number of segments removed was studied, partly with standard percentages (5.26%) per segment (method I), and partly with a percentage per segment in the affected area, calculated from the preoperative regional lung function tests using 133xenon. This latter test was carried out with a mobile apparatus using four detectors with tubular colimators and from the anterior surface of the thorax. Both methods of calculation gave, for the material as a whole, good agreement between the postoperative (measured) and the calculated values. However, with regard to certain patients, the regional lung function tests gave important information on preoperative reduced function in the affected area. In these patients, method No. II was by far the best for prediction of the postoperative values.