Thyrotoxicosis is often perceived as a reversible disorder without long-term consequences, perhaps because of the availability of effective treatments, but recent evidence suggests that there may, in fact, be adverse outcomes. Long-term follow-up studies have revealed increased mortality from cardiovascular and cerebrovascular disease in those with a past history of overt hyperthyroidism treated with radioiodine as well as in those with subclinical hyperthyroidism indicated by a low serum TSH concentration. Thyroid hormones exert direct effects on the myocardium as well as the systemic vasculature predisposing to dysrhythmias, especially supraventricular. Effects of thyroid hormones on the autonomic nervous system may also contribute to arrhythmogenesis. Atrial fibrillation is a recognized complication of hyperthyroidism. that predisposes to embolic events. Development of atrial fibrillation, together with other supraventricular dysrhythmias (both clinically obvious and those detected only by Holter monitoring) in those with hyperthyroidism may account for increased vascular mortality. Improved detection of supraventricular dysrhythmias and therapeutic intervention (e.g. anticoagulants, antiarrhythmics) may improve the long-term vascular prognosis, but their role remains to be established in large therapeutic trials.