Air cysts of the lung which produce symptoms sufficient to call clinical attention to their existence must be considered an uncommon condition. Croswell and King1 have been able to find reports of only 121 cases, including the 108 case reports compiled by Koontz.2 The fact is, however, that although rarely encountered clinically, cystic disease of the lung not infrequently presents important diagnostic and therapeutic problems. A report, together with a discussion of a group of cases illustrating the clinical features as well as the diagnostic and therapeutic problems which they presented, is therefore warranted. Most of the publications on the subject have been essentially from the standpoint of the pathology and pathogenesis. In occasional instances case reports have been published dealing with the results of treatment. This paper aims to illustrate some important types of cystic disease of the lung, especially from the point of view of clinical