In enlargement of one or more of the cardiac chambers, the roentgen cardiac silhouette deviates from the normal in a predictable fashion, as described in standard radiologic texts. The classical changes are primarily due to local expansion of the part of the cardiac contour formed by the chamber under consideration. As the chamber continues to enlarge, however, in some cases secondary changes in the shape of the heart due to displacement of other cardiac chambers may lead the unwary to misinterpret the roentgen findings. In this paper some examples of these changes will be illustrated by comparing biplane angiocardiograms with plain films of the chest, and some guide lines for avoiding misinterpretation will be discussed. Case Material Displacement of Other Cardiovascular Structures by the Greatly Enlarged-Left Atrium Case I: G. D., a 59-year-old woman with severe rheumatic mitral stenosis and regurgitation. The angiocardiogram confirms the presence of marked left atrial enlargement (Fig. 1) and shows forward displacement of the left ventricular outflow tract and aortic root by the left atrium, without causing misinterpretation of the plain films. Case II: L. H., a 50-year-old male with rheumatic mitral stenosis and insufficiency. Chest films (Fig. 2, A) appeared to show considerable right atrial enlargement as well as marked left atrial enlargement. A left ventricular angiocardiogram (Fig. 2, B) demonstrates mitral regurgitation and confirms the presence of left atrial enlargement, though the left atrium does not extend to the right heart border. Though the right ventricular (36/0–10) and right atrial (mean 9 mm Hg) pressures were elevated, at surgery for mitral valve replacement the right atrium did not appear particularly enlarged, but was considerably displaced, being stretched crescent-like around the huge left atrium. Case III: R. G., a 44-year-old woman with rheumatic heart disease who had been subjected to closed valvotomy two years earlier for mitral stenosis. Chest roentgenograms (Fig. 3) revealed enlargement of the right ventricle and left atrium, and apparent marked enlargement of the right atrium. Cardiac catheterization disclosed mitral restenosis, and left ventricular angiography showed severe mitral regurgitation. Right ventricular (65/0–7) and right atrial (7 mm Hg) pressures were elevated. However, at operation for mitral valve replacement, the right atrium appeared little if any enlarged but was displaced by the large left atrium. Displacement of Other Cardiac Chambers by the Enlarged Right Ventricle Case IV: S. J., a 54-year-old woman with an endocardial cushion defect, including a defect in the atrial septum and cleft mitral valve without regurgitation. There were pulmonary hypertension and bidirectional shunting. Chest radiographs (Fig. 4, A) were considered to demonstrate left ventricular enlargement as well as marked right ventricular enlargement.