For two and a half years we observed a 3 year old boy, whose entire left thoracic cavity was filled with a balloon-like cyst. His condition varied from that of threatened asphyxia to that of restful normal breathing, relief being effected only by aspirating from 8,000 to 10,000 cc. of air from this sac every four to six hours. During the periods of relief, he was able to take nourishment and rest as if in preparation for the exhausting symptoms that were sure to return. Few large solitary air cysts or "balloon cysts" of the lung have been reported in the American literature. This is shown by Koontz,1who made an extensive review in 1925 and tabulated 108 cases; of these, only 7 were of the large solitary type. We have found twelve additional cases reported in the American literature since 1925. The case reported here brings the total