20 patients with hypothyroid cardiomyopathy were evaluated by M-mode echocardiography. The mean interventricular septum (IVSd) thickness was 1.94 cm before treatment and was reduced to 1.21 cm after 4-6 months of thyroxin therapy. The mean left ventricular posterior wall (LVPWd) thickness was 1.14 cm before and 1.05 cm after treatment. The IVS/LVPW ratio decreased from 1.7 to 1.15 within 4-6 months. Pericardial effusion was demonstrated in 15 of the patients, but disappeared during thyroxin therapy. The study confirms the importance of examining for hypothyreoidism in the case of patients with symptoms of heart failure of unknown cause, and of patients with echocardiographic evidence of asymmetric septal hypertrophy (ASH) and idiopathic hypertrophic subaortic stenosis (IHSS).