To evaluate latent left ventricular dysfunction in patients with Graves' disease, hemodynamic response to isometric exercise were examined in 93 patients and 18 controls by impedance cardiography. All patients were treated with anti-thyroid drugs and 64 patients were stll clinically thyrotxic with elevated triiodothyronine (T3) in serum. The remaining 29 patients were clinically free of thyrotoxicosis with normal serum T3. Thirty three of 93 patients received beta-bloccker.The distinct elevation of cardiac index (CI) , associated with elevation of cardiac work index (CWI) and total peripheral resistance index (TPRI) , was observed during the exercise in control group. Values for CWI and TPRI were apparently elevated in all patients studied, and the magnitude of the alterations in CWI and TPRI before and during the exercise was almost the same as that observed in control group. However, the CI values were almost unchanged before and during the exercise in hyperthyroid patients without beta-blockade, suggesting the impaired cardiac reserve in hyperthyroidism.Moreover, it was noteworthy that CI values before and during the exercise were also unchanged even in euthyroid patients without beta-blockade. This result supports the contention that a biochemical euthroid state may be achieved many weeks before normalization of contractile response to exercise.There was a significant correlation between serum T3 and CI at rest in 60 patients without beta-blockade, but this correlation was lost in 33 patients, who received beta-blocker. The beta-blocker induced reduction of the resting CI was evident in hyperthyroid patients, and this reduction of CI values was also observed in these patients with beta-blocker during exercise,showing negative inotropic effects of the beta-adrenoreceptor blocking drugs. Thus, beta-blocker may be beneficial in lowering heart rate in severe hyperthyroidism, but the beta-blockade should be avoided in euthyroid patients, who showed the delayed recovery of left ventricular function after antithyroid treatment.