Increased radiolucency of one lung associated with decreased prominence of its hilar and pulmonary vascular markings is observed most commonly in obstructive emphysema due to occlusion of a main or lobar bronchus by a mass or foreign body and in compensatory emphysema following collapse of a pulmonary lobe or segment. Recently, however, there have appeared several reports describing a condition in which the roentgen findings simulate obstructive or compensatory emphysema but which proves on further roentgen examination to be neither. The first comprehensive analysis of a group of such patients was made by Macleod, who reported 9 cases under the title Abnormal Transradiancy of One Lung (2). Later Dornhorst, Heaf, and Semple (1), designating the process as “unilateral emphysema,” described 4 additional examples and reported follow-up studies on 5 of Macleod's original cases. Neither of these reports identifies the disease as one of the recognized forms of pulmonary emphysema. Actually the first case, in a b...