Cardiac symptoms have a prominent position in hyperthyroidism. The basic haemodynamic change is an increased cardiac output due to an increased stroke volume and an increased heart rate. Dyspnoea and palpitations are frequent symptoms. ECG-changes such as atrial fibrillation, sinus tachycardia, prolonged atrioventricular conduction and ST-segment changes are seen, but such changes are unspecific. Chest X-ray and echocardiographic studies are often normal, but cardiomegaly and ventricular hypertrophy are seen. The cardiac and non-cardiac symptoms of hyperthyroidism may in part be related to an increased sympathoadrenal activity. The thyroid hormones also exert a direct effect on the heart. Treatment includes specific anti-thyroid medication, beta-blocking agents, digoxin, diuretics and possibly anticoagulant therapy. Congestive heart failure, ventricular hypertrophy and arrhythmia are most commonly seen if other cardiac abnormalities are already present. The existence of thyrotoxic cardiomyopathy as an independent disease is illustrated by two case histories, which also demonstrate the reversibility of this condition.