AGENSIS of the lung is a rare condition as shown by a careful review of the literature. A certain amount of confusion has arisen, however from the loose usage of the terms "agenesis" and "aplasia." "Agenesis" has been used for conditions ranging from complete absence of the lung and main bronchus to those in which a variable amount of pulmonary tissue accompanies a rudimentary bronchus. It would be advisable to conform to the more precise usage of the terms and to use "agenesis" for a failure of development and "aplasia" for incomplete development. Schneider1 has classified congenital defects of this kind into three main types: (1) true agenesis with no trace of the lung, bronchus or vascular supply on the affected side; (2) a type characterized by small outpocketing from the trachea with a rudimentary bronchus but no pulmonary tissue present, and (3) a type in which a fully