Eleven children with primary hypothyroidism were studied by echocardiography. Eight patients had pericardial effusion. Systolic time intervals (preejection period-ejection time ratio and left ventricular isovolumic contraction time) were suggestive of abnormal myocardial function. Asymmetric septal hypertrophy without left ventricular out-flow obstruction (ventricular septal-left ventricular posterior wall diastolic thickness ratio, greater than 1.3) was present in two hypothyroid children. After full replacement therapy, these cardiac abnormalities reversed to normal. Our data suggest that the myopericardial response to hypothyroidism in children is similar to that seen in adults.