The clinical features of congenital absence of the pectoralis major are well recognised and descriptions of the condition can be found in most standard text-books of surgery.The radiographic appearances are not so well-known and may cause some difficulties in diagnosis. These difficulties are unlikely to arise if the patient has been examined clinically by the radiologist. However, under many circumstances,e.g., in mass miniature radiography surveys, pre-radiographic clinical examination is not possible and difficulties in radiological diagnosis are consequently far greater.The pectoralis major may completely fail to develop or, more commonly, is only partially developed. Several authors have recorded clinical accounts of this condition (Richardson, 1940; Parhad, 1939; Rosenthal, 1940; Jokl, 1939), but apart from Rosenthal's report little attention has been paid to the radiological appearances. This is not altogether surprising in view of the ease of clinical diagnosis.The pectoralis major is a thick triangular muscle situated in the upper and front part of the chest. It has two origins:—(a) The clavicular head which arises from the anterior surface of the sternal half of the clavicle, and(b) The sterno-costal portion which arises from half the breadth of the anterior surface of the sternum as low down as the attachment of the cartilage of the sixth or seventh rib and from the cartilages of all the true ribs with the exception of the first or seventh rib and from the aponeurosis of the external oblique muscle (Gray, 1944).