Six cases of atelectasis developing in the unopened contralateral lung immediately after thoracotomy in the lateral position were presented, and its cause and treatment were discussed. Atelectasis due to retention of secretion showed various atelectatic X-ray shadows which differed in size by the segment or lobe unit. On the other hand, cases of atelectasis whose occurrence was surmised to be related to the lateral position and anesthesia showed mainly infiltrative shadows which extended from the outer region of the middle-lower lung field to the basal segments of the lung, although some cases were accompanied by atelectatic shadows of various size in terms of the segment or lobe unit. Therefore in order to prevent atelectasis, it is desirable to move sputum by aspiration and appropriately apply ventilation, intermittent pressurization and PEEP, while paying careful attention to the unopened lung. In the treatment of atelectasis, selective endobronchial pressurization using a bronchofiberscope equipped with a cuff was effective. It was also very effective to place an endotracheal tube for ordinary anesthesia near the target bronchus with the aid of a bronchofiberscope and perform selective endobronchial pressurization via that tube.