Pulmonary atelectasis is a common complication following surgery under general anesthesia. However, collapse during anesthesia and surgery is rare, and usually is not diagnosed until the surgical procedure is well under way. Total or segmental lung collapses are usually resulted from the obstruction of bronchial pathway by secretions such as mucus, blood and pus etc. We experienced acute lung collapse of right upper lobe during left thoracotomy. We assumed that the cause of the atelectasis was an obstruction of right superior lobar bronchus by mucus. The possible cause of lung collapse is described.