Introduction The effusion of chyle into a pleural space from a damaged or diseased thoracic duct is now a well-established entity. The development of chylothorax has been recognized for centuries76; yet only in the past decade have satisfactory surgical methods been devised for use in cases which have been refractory to conservative management. The various pathological circumstances which give rise to intrathoracic effusion of chyle are well documented, and so the etiology is clearly defined in most instances. However, the occurrence of chylothorax in early infancy, in the absence of other demonstrable disease and without apparent birth trauma, suggests the existence of a congenital malformation of the thoracic duct. Considered in the light of existing knowledge of the embryology and anatomy of the thoracic duct, combined with its appearance in chylothorax as seen at operation or at necropsy, there is formidable argument for accepting the idea of malformation as