Fifteen selected hypothyroid patients without symptoms or signs of cardiovascular disease and an equal number of matched control subjects underwent simultaneous recording of electrocardiogram and phono-, apex-, and echocardiography to assess dynamic systolic and diastolic left ventricular function. Both the systolic preejection period and the isovolumic relaxation period were significantly increased in the hypothyroid group. However, whereas the rate of myocardial contraction, assessed from the echocardiograph of the left ventricular posterior wall, was identical in patients and control subjects, the diastolic thinning rate of the muscle was markedly slowed in the hypothyroid individuals. The abnormalities demonstrated were in the main completely reversed after 3 months of T4 therapy. These results demonstrate a relatively selective and readily reversible disturbance of the rate of myocardial relaxation in hypothyroidism, suggesting an intrinsic abnormality of cardiac muscle. This allows an intriguing parallel to be drawn with the delayed relaxation phase of voluntary muscle contraction, long recognized as a direct measure of tissue thyroid function in hypothyroidism. The abnormality of diastolic function we have described is of similar character to that found in patients with other cardiomyopathies and which has been shown to be a major cause of disturbance of global cardiac action.