For revealing whether cardiac performance is changed in subclinical hypothyroidism, the left ventricular ejection fraction (LVEF), a value characterizing left ventricular function, was studied in this clinical state. The results were compared to those obtained in euthyroid subjects and patients with overt hypothyroidism. In a part of subclinical cases, there is a subnormal resting LVEF, which on isometric exercise, increases, but to a lesser extent than in euthyroid controls. In overt hypothyroidism resting LVEF is considerably lower than in euthyroid and subclinically hypothyroid subjects and its value does not respond to exercise. There has been evidence that heterogeneous subclinical hypothyroidism is an intermediate state between euthyroidism and overt hypothyroidism; the majority of patients are euthyroid, a part of them, however, based on systolic time intervals and LVEF studies, are suspect of developing 'tissue' hypothyroidism, although their serum thyroid hormone level is still within normal limits.