After a lobectomy the apearance of the chest roentgenogram may return so nearly to normal that it isfrequently very difficult to tell which lobe has been moved without refering to the thoracic surgeon's record. Thereriew of literature failed to disclose previous articles concerning the differential diagnosis between upper andlower lobectomy. Clues of a lobectomy may be found in the rib cage, hilar shadows, pleura and disphragms, but they do not specifically incidate which lobe has been removed. In the present study we anlaysed anatomico-spatialchanges of the pulmonary basal arteries, hilar point, vascular redistribution, diaphragm and rib cage on the plainchest films taken before and after a lobectomy in 33 cases seen at the Dept. of Radiology, St. Mary's Hospoital, Catholic Medical College. Firstly we observed the pulmonary basal artery after a lobectomy on plain chest film. In 12 cases of upper lobectomy the pulmonary basal artery was easily identified in every case. However in all of 21cases of lower lobectomy, the pulmonary basal artery was not identified. Next, a shift of the hilar point waschecked after a lobectomy. Regarding to vascular redistribution, the blood vessels was counted at upper and lowerlung fields by simon's method before and after a lobectomy, respectively. Finally, the level of the diaphragm wascompared in the pre. and post-opeative films and resected rib was scrutinized. The present study revealed that themost reliable sign to indicate specifically which lobe has been resected is persistence or disappearance of thepulmonary basal artery. Then i.e. in upper lobectomy the pulmonary basal artery was easily identified, but inlower lobectomy the pulmoanry basal artery was not dectable. Other findings such as vascular redistribution,elevated diaphragm and resected rib were not specific.