A group of infants, affected by congenital hypothyroidism diagnosed through the neonatal screening program, was investigated with echocardiography to detect the presence of pericardial effusion. We studied the relationship between the effusion and the etiology of hypothyroidism, established through thyroid scintiscanning. Our data show a high prevalence of effusion in hypothyroid patients, without other clinical signs of cardiac involvement as well as a relationship between the etiology of hypothyroidism and the presence of effusion. This seems to be much more frequent in those forms which can imply a more severe hormonal defect, particularly during fetal life (agenesis/dyshormonogenesis). Furthermore, the high prevalence of pericardial effusion suggests to start the L-T4 replacement therapy with lower dosages as commonly advised, in order to avoid a cardiac involvement.