Spiroergometry might be applicable to detect alterations of cardiopulmonary functions related to hyperthyroidism. Thus, cardiac and respiratory changes as well as work capacity in hyperthroid female patients were to be assessed with the help of the Cardiopulmonal Exercise Test System. Twelve female hyperthyroid patients with Graves' disease of whom all were controlled in euthyroidism, were examined. Eighteen euthyroid female patients in whom intracardiac catheter examination ruled out cardiopulmonary disease served as controls. The anaerobic threshold was determined by means of the V-slope method. An echocardiography was performed in all patients. Ergometry was performed in a semisupine position using a continuous ramp protocol of 20 watt/min. A markedly reduced work capacity, and a high heart rate in rest and exercise were found. In the ratio heart rate/oxygen uptake a lower rise (p = 0.001) due to a decreased growth in the heart rate was noticed. Regarding the pulmonary system a decreased tidal volume in hyperthyroidism (p = 0.021), and a higher breathing frequency (p = 0.003) were recognized, as well as an impaired oxygen consumption, in comparison with the euthyroid state. Also, echocardiographically an increased cardiac index (p = 0.008) and a markedly reduced stroke volume (p = 0.005) in comparison to the control group were observed. Heart rate, work capacity, oxygen uptake, and the ratio heart rate to oxygen uptake were normalized in euthyroidism. With the help of the CPX-System noninvasive measure of marked cardiopulmonary changes in hyperthyroidism are possible, especially the lower growth of the heart rate in exercise, which might be the limiting factor of work capacity.