The purpose of this study is to investigate the effects of thyroid hormone on the left ventricular(LV) volume and function in man with untreated hyperthyroidism and to determine the effects of successful therapy for thyrotoxicosis on the ventricular pathophysiology. In the precent study, equilibrium ralhianuclide cardiac angiography was performed and LV volume index, ejection phase indexes of LV performance, serum thyroid hormone levels and other hemodynamic parameters were measured in 28 normal subjects and 39 patients with hyperthyroidism before treatment and again every 4 weeks for the first 2 months after the initiation of effective therapy. The result obtained were as follows; 1) In the untreated hyperthyroid state heart rate, blood volume, cardiac index and stroke volume index(97±14 beats/min, 73.5±11.8 ml/kg, 6.9±1.4 l/min/m2 and 77.6±13.8 ml/m2, respectively) were increased significantly compared to those in normal control(74±12beats/min, 65.6±14.8 ml/kg, 3.8±1.2 l/min/m2 and 56.6±13.2 ml/m2 respectively). (Mean±SD). 2) There was a significant increase in LV end-diastolic volume index in patients with hyperthyroidism(30.5±7.5 for hyperthyroid group compared to a normal control of 22.2±6.5;P〈0.001), whereas end-systolic volume index remained unchanged 9.6±3.6 and 8.8±3.3 respectively). 3) In patients with hyperthyroidism, LV ejection fraction was 70.0±5.6%, fractional shortening 32.9±5.1%, mean velocity of circumferential fiber shortening(mean Vcf) 1.34±0.31 circ/sec and maximum ejection rate 3.47±0.80. All the ejection phase indexes were significantly greater than those in normal control(65.2±5.7%, 28.8±3.2%, 0.88±0.37 circ/sec and 2.27±0.50, respectively; p〈0.001). 4) Effective therapy produced significant decrease in all the values of serum thyroid hormone concenrations(p〈0.001), hemodynamic parameters(p〈0.001), end-diastolic volume index(p〈0.01) and ejection phase indexes of LV contractility in patients with hyperthyroidism and after one to two months, when the patients were