Atrial fibrillation (AF) is the commonest dysrhythmia (after sinus tachycardia) complicating hyperthyroidism. We have previously reported increased vascular mortality in hyperthyroidism and postulate this reflects the prevalence of AF. We recruited a consecutive series of 425 subjects with overt hyperthyroidism (n=334 females, 91 males) and determined the prevalence and outcome of AF. All had 12 lead ECG and 24h Holter monitoring before and after antithyroid treatment with thionamides and/or radioiodine. Amongst the cohort of 425, the prevalence of AF was 8.0% versus 1.0% in 425 age- and sex-matched controls (P<0.0001). Of the 35 subjects identified with AF, 20 had new persistent AF, 10 had known persistent AF and 5 had new paroxysmal AF (PAF). The mean age of hyperthyroid subjects with AF was less than those without (69 vs 49.5 y, P<0.0001) and they more commonly had a history of vascular disease (51.4 vs 21.8%, P<0.0001). Independent risk factors for development of AF were age, vascular disease, systolic and diastolic BP and presence of LVH on ECG. Over a mean follow-up of 32.6 weeks, 10 of 35 subjects returned to sinus rhythm (SR) spontaneously, 2 had successful cardioversion and 23 remain in AF or died. Of 20 with new persistent AF, 13 remain in AF, 1 died and 1 is in SR. Of those with known AF, 6 remain in AF, 3 died and 1 is in SR. All 5 cases of PAF are in SR. During a mean follow-up of 147 weeks, 17 of the hyperthyroid cohort died (4 amongst cases of AF), compared with 4 deaths amongst 425 controls (P=0.003). Independent predictors of death were male sex, age, previous vascular disease and AF (P=0.01). AF remains a common complication of hyperthyroidism and restoration of SR is uncommon (34%). AF is an independent predictor of mortality amongst hyperthyroid subjects.