Pulmonary function studies today are generally accepted as an integral part of the evaluation of poor-risk patients who are to have pulmonary surgery. The effect of various pulmonary surgery on lung function was investigated in 54 patients in whom comprehensive lung function test were performed before and between 2 months and 14 months after operation at the Department of Thoracic Surgery, Seoul National University Hospital. According to the result of analysis, the effect of pulmonary resection on forced flow rate was keeping with the change of lung volume, and the preoperative level of ventilatory function plays a major role in determining postoperative loss of functioning lung. Although all measures of expiratory flow [FVC, FEV1, FEFO.2-1.2, MEF50, FEF25-75] have the same percentage of reproducibility, but FEV1 shows most sensitive, reliable linear correlation with the functioning pulmonary tissue loss than other parameters. The linear regression lines derived from the correlation between preoperative [X] and postoperative [Y] FEV1 on various surgical procedures were as follows: 1. Y = 0.57X 0.03. in pneumonectomy group of lung cancer[r=0.84]. 2. Y = 0.56X + 0.33. in lobectomy group of lung cancer[r=0.79]. 3. Y = 0.69X + 0.25. in lobectomy group of pulmonary infection[r=0.91].