The authors describe the clinical characteristics and response to therapy of seven patients with hyperthyroidism, dilated cardiomyopathy, and low-output cardiac failure. All patients (4 women and 3 men, age 47 +/- 4 years, mean +/- standard error of the mean) were admitted with the primary diagnosis of congestive heart failure. The cause of hyperthyroidism was Graves' disease in six patients, and toxic multinodular goiter in one. On admission, the mean serum T4 was 21 +/- 1 microgram/dL and mean serum T3:411 +/- 77 ng/mL, and serum thyroid-stimulating hormone was suppressed ( < 0.03 microU/mL) in all patients. Two-dimensional echocardiogram showed biventricular or four chamber dilatation and impaired left ventricular performance. Therapy of heart failure and hyperthyroidism resulted in rapid clinical improvement. During follow-up (5 months to 9 years), left ventricular ejection fraction improved from a mean of 28% to a mean ejection fraction of 55% (P < 0.01). Resolution of dilated cardiomyopathy with normalization of systolic function was achieved in five patients, and improvement from severe to mild left ventricular dysfunction was observed in two patients. We conclude that some patients with hyperthyroidism may have a reversible form of dilated cardiomyopathy and "low-output failure." Assessment of thyroid hormone status in patients with heart failure might permit the identification of patients with dilated cardiomyopathy and thyrotoxicosis who are likely to have reversible cardia dysfunction.