This study of individuals with familial and sporadically occurring 46 XX or XY Turner phenotype documented a wide range of right- and left-sided cardiovascular abnormalities and a previously unreported eccentric hypertrophy of the left ventricle.A mother and five of her seven children had abnormal cardiovascular findings. Five had an abnormal electrocardiogram with frontal ÂQRS of −60° to ±180° and rS or rsr′ in V1and rS, qrS, or qRS in V5. Catheterization demonstrated the following anomalies: coarctation of the aorta in three, valvular aortic stenosis in one, pulmonary valvular insufficiency with atrial septal defect in one, and pulmonary arterial branch stenosis in one child. All six had a similar abnormality of the left ventricle on angiocardiography. During systole and also in diastole the cavity was encroached on in its superolateral and posteroinferior aspects. Septal hypertrophy altered right ventricular contour in two.A similarly abnormal electrocardiogram and left ventricle were found in four unrelated individuals with the XX, XY Turner phenotype. Three had pulmonary stenosis; in two there was an associated septal defect. The fourth, with no associated cardiac defect, died in heart failure at 5 months of age. At necropsy she had marked eccentric biventricular hypertrophy, chiefly involving the left ventricle so that the chamber was reduced to a slitlike cavity. The hypertrophied septum bulged into the right ventricular outflow tract.A number of cardiovascular anomalies occur in familial and sporadic instances of this syndrome; eccentric ventricular hypertrophy recognizable by an electrocardiographic abnormality seems to be a distinctive cardiac lesion in the XX, XY Turner phenotype. Except for the unusual and, we believe, characteristic ECG, there was no clinical clue on physical examination or cardiac series of chest roentgenograms to suggest the presence of eccentric left ventricular hypertrophy. We recommend that selective left ventricular angiocardiography be performed when patients with the Turner phenotype undergo diagnostic cardiac catheterization, especially when the frontal ÂQRS is superiorly directed.