Maximal expiratory flow-volume [MEFV] curves were studied in 22 patients who underwent pneumonectomy with various pulmonary lesions, such as lung cancer, bronchiectasis and tuberculosis etc, at the preoperative stage and 3 week, 4 month and 12 month after pneumonectomy for the analysis of the reduction and progressive improvement of postoperative ventilatory function. And the factors affecting them like as age difference and the site of pneumonectomy were also analyzed. From these curves peak flow rate [PF R], maximal expiratory flows at 25% and 50% of expired forced vital capacity [V25, V50] and forced vital capacity [FVC] were obtained. In addition, partial pressure of oxygen and carbon dioxide in arterial blood were measured. The results were as follows; 1. The mixed type, especially obstructive type of ventilatory impairment was observed at 3 week after operation. For 1 year of postpneumonectomy FVC was increased by 12.3% of predicted compared to 2.6% of predicted V50. 2. The improvement of FVC during 1 year of postpneumonectomy showed decreasing tendency with the increase of age but the changes of V25 and V50 were unremarkable. 3. The differences of immediate postoperative reduction and progressive improvement of ventilatory capacity after right and left pneumonectomy were analyzed. The reduction of V50, V25 and FVC at 3 week of postoperation were greater in patients with right pneumonectomy [20.9%, 18.2% and 26.2% of predicted] than in patients with left pneumonectomy 16.5%, 18.2% and 18.1%]. But there was no significant difference of these values at 12 month after pneumonectomy. 4. The partial pressure of oxygen in arterial blood [PaO2] was decreased by 13.6 mmHg at 3 week after pneumonectomy compared to the preoperative stage but returned to the normal range within 4 month after pneumonectomy. However, PaCO2 was within the normal range during 1 year of postoperation. [KTCS 1983;4:539-546]