The purpose of this study is to investigate the effects of thyroid hormone on the left ventricular(LV) volume arid function in man with untreated hyperthyroidism and to determine the effects of successful therapy for thyrotoxicosis on the ventricular pathophysiology. In the present study, equilibrium radionuclide 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 results obtained were as follows; 1) In the untreated hyperthyroid state heart rate, blood volume, cardiac index and stroke volume index($97{pm}14$ beats/min, $73.5{pm}11.8ml/kg,;6.9{pm}1.4;l/min/m^2$ and $77.6{pm}13.8ml/m^2$, respectively) were increased significantly compared to those in normal control($74{pm}12beats/min$, $66.6{pm}14.8ml/kg,;3.8{pm}1.2;l/min/m^2$ and $56.6{pm}13.2ml/m^2$ respectively). $(Mean{pm}SD)$ 2) There was a significant increase in LV end-diastolic volume index in patients with hyperthyroidism ($30.5{pm}7.5$ for hyperthyroid group compared to a normal control of $22.2{pm}6.5$; p<0.001), whereas end-systolic volume index remained unchanged $9.6{pm}3.6;and;8.8{pm}3.3$ respectively).3) In patients with hyperthyroidism, LV ejection fraction was $70.0{pm}5.6%$, fractional shortening $32.9{pm}5.1%$, mean velocity of circumferential fiber shortening(mean Vcf) $1.34{pm}0.31$ circ/sec and maximum ejection rate $3.47{pm}0.80$. All the ejection phase indexes were significantly greater than those in normal control($65.2{pm}5.7%,;28.8{pm}3.2%,;0.88{pm}0.37$ circ/see and $2.27{pm}0.50$, respectively; p<0.001). 4) Effective therapy produced significant decrease in all the values of serum thyroid hormone concentrations(p<0.00l), 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 euthyroid, these measurements were in the range of normal. 5) A significant linear correlation between mean Vcf and serum thyroxine level(r=0.63, p<0.001) as well as between mean Vcf and serum triiodothyronine level(r=0.62, p<0.001) was found. The lesser degree of correlation was also noted between other ejection phase indexes and serum thyroid hormone concentrations. The results indicate that the major effects of excess thyroid hormone on the LV in human beings with hyperthyroidism are an enhancement of LV function and an increase in LV enddiastolic volume and that these effects cause predictable reversible cardiac alteration which are changed dramatically and immediately after effective therapy.