Right and left heart volumes, ejection fractions and ventricular outputs were determined from biplane cineangiocardiograms in infants with symptomatic coarctation of the aorta and correlated with clinical and hemodynamic alterations. Patients were divided into two age groups: group 1, aged 3.5 to 14 days and group 2, 5 weeks to 7 1/2 months. Infants in group 1 had severe depressions of left ventricular ejection fraction and output associated with normal left ventricular size. The massive cardiomegaly in these infants results from right heart enlargement secondary to left-to-right atrial shunting, and pulmonary hypertensive right heart failure, and possibly a more distensible right than left ventricle. Infants in group 2 also have right, and to a lesser degree, left heart enlargement. Group 2 patients differ from group 1 infants in having less impairment of left heart pump function and significant left ventricular myocardial hypertrophy. Echocardiographic measurements of left ventricular pump function are normal or increased in postoperative patients. Thus alterations of left ventricular function in infants with symptomatic coarctation appear to be largely afterload related and do not indicate permanent impairment of left ventricular contractile function.