The short-term effects of L-thyroxine (100 micrograms/day, 10 patients) and placebo (10 patients) on idiopathic dilated cardiomyopathy were compared. Before and at the end of the treatment, a hemodynamic study was performed in the control state and during dobutamine infusion. A cardiopulmonary exercise test was also performed with hemodynamic monitoring. An echocardiogram was recorded in the control state and during acute changes of left ventricular afterload. Plasma levels of triiodothyronine, thyroxine, thyroid-stimulating hormone and norepinephrine were measured. Placebo was ineffective. After administration of L-thyroxine all patients had normal thyroid function. The increase in left ventricular ejection fraction and the rightward shift of the slope of left ventricular ejection fraction/end-systolic stress relation (p < 0.05) indicated an improvement in the cardiac inotropic state. This proved to be independent of adrenergic influences by the unchanged beta 1 response to dobutamine. A decrease in resting systemic vascular resistances and an increase in cardiac output (p < 0.05) were also observed. Cardiopulmonary effort parameters improved (p < 0.05) without hemodynamic changes at peak exercise. It is concluded that L-thyroxine short-term administration improves cardiac and exercise performance in patients with chronic heart failure, without modifying the adrenergic support to the heart and the circulatory parameters at peak exercise.