Thirty-nine patients with untreated hypothyroidism have been examined using echocardiography for the presence of a pericardial effusion. Effusions were present in twelve patients who tended to be more severely hypothyroid. Plasma creatine phosphokinase and lactate dehydrogenase levels were higher in the presence of an effusion. Nine were reinvestigated during thyroxine replacement therapy and the effusions did not disappear until thyroid function tests had returned to normal. There were no specific electrocardiographic changes associated with the presence of an effusion which could be associated with a normal cardiac silhouette on a standard P.A. chest X-ray.