Studies of regional pulmonary function using radioactive 133xenon gas and spirometric tests (forced vital capacity and forced expiratory volume in the first second) were performed before and after unilateral pulmonary resection for cancer of the lung. Ninety-one patients were evaluated; 47 underwent total pneumonectomy, and 44 underwent lobectomy. The postoperative serial evaluations were classified into short-term and long-term studies (less than or more than three months, respectively). The preoperative and postoperative data were utilized to derive formulas for predicting an estimate of the overall functional loss after pulmonary resection based on the number of segments removed. The correlation between the predicted and measured postoperative values was good for resections involving more than three segments (r = 0.83). Prediction for smaller resections was unreliable. While both regional and overall pulmonary functions were relatively stable after pneumonectomy, there was a disproportionate early loss, followed by significant functional improvement with time following lobectomy. The anticipation of and preparation for this early loss of function may be crucial in the treatment of these patients.