Atrial septal defects have been shown to be one of the commonest single congenital cardiac abnormalities. Many of these defects are well borne during infancy and childhood and do not manifest themselves until adult life. The pattern of symptomatology then is one of progressive reduction in myocardial reserve, followed by the gradual appearance of cyanosis and, finally, death due to a combination of right heart failure and increasing systemic anoxia. On the other hand, as reported previously by Downing and Goldberg,2a very sizable number of infants suffer severely and die from this disease during the early months of life. Other children survive but exhibit marked degrees of physical underdevelopment due to an insufficient systemic circulation. A somewhat larger number experience sharp limitations in myocardial reserve, being unable to "keep up" with their fellows on maximal physical exertion, although quite comfortable at rest. Abnormalities of development of the interatrial