Myocardial contractility was estimated in 27 patients with hyperthyroidism (10 men and 17 women, aged from 16 to 38) by echocardiography. Echocardiographic examination was performed before and on the 7th day after the subtotal thyroidectomy. Thyroid functions (PBI, T3, and T4) decreased significantly after the operation (p less than 0.01). Heart rate and systolic blood pressure showed a slight decrease (N.S.), while diastolic blood pressure showed a slight increase (N.S.), while diastolic blood pressure showed a slight increase (N.S.). Mean AWVs decreased from 35.8 +/- 7.0 mm/sec to 27.6 +/- 5.7 mm/sec and correlated wtih T3 (r=0.4008, p less than 0.05). Mean PWV's also decreased from 53.2 +/- 12.2 mm/sec to 45.2 +/- 11.4 mm/aec and correlated with T3 (r=0.5237, p less than 0.02). DDR decreased from 108.0 +/- 36.8 mm/sec to 85.7 +/- 31.7 mm/sec and correlated wtih T3 (r=0.4008, p less than 0.05). Above results suggest that enhanced myocardial contractility and elevated rate of ventricular filling in hyperthyroidism are suppressed after the operation, and that echocardiography is useful for the evaluation of the clinical course after the operation.