The relationships of cardiac size and contour to respiratory dynamics in infants and the newborn who are suffering from the “bronchiolitic syndrome” are well known. Briefly, during the height of this inflammatory disease there are marked overventilation of the thorax with depression and flattening of the diaphragm, an increased anteroposterior thoracic diameter, bulging of the sternal plate anteriorly, and frequently widening of the rib interspaces. The cardiothoracic ratio during this period is markedly reduced, and there is actually a reduction in the overall roentgen size of the heart. There may also be changes in its configuration. If criteria for specific chamber enlargement depend either on the actual outline and configuration of the heart or upon its relation to the chest wall (1), these relationships are seriously disturbed merely because of change in the thoracic contour and not necessarily as a result of any intrinsic cardiac abnormality. These changes are well illustrated in Figure 1, A, which shows an infant chest during an attack of bronchiolitis. In Figure 1, B the chest is seen following recovery. The marked difference in cardiac contour is apparent. Several authors have realized the difficulty of accurate chamber analysis in infants. Sussman and Jacobson (2), in a well controlled study, showed the difficulty of evaluation of right ventricular enlargement, but their series did not include infants. Others have written on the relation of emphysema and left-to-right shunt (3) but have not emphasized the relationship of heart size and configuration to these changes. Friedman and Ash (4) specifically mentioned overdistention of the lungs in 8 of 11 infants with a persistent atrioventricular canal. Maxwell (5) discussed the clinical appearance of Harrison's grooves in a patient with ventricular septal defect and relates it to the dyspnea. Still other writers have commented on the problems involved in particular chamber analysis in various congenital heart diseases (6–8). Recently, Lambert, Perez, and Vlad (9) discussed emphysema in infants with congenital heart disease and demonstrated a significant lowering of the diaphragmatic level as compared with normal controls. This was associated with a changing cardiothoracic diameter and inferior displacement of the liver. As one analyzes films of infants with severe cardiac abnormalities, either those demonstrating a large left-to-right shunt or, occasionally, those with a marked pulmonic stenosis, the frequency of over-aeration is striking.