Twenty patients with thyrotoxic Basedow's disease complicated by atrial fibrillation lasting more than one month despite treatment with antithyroidal drugs were treated with radioiodine supplemented with an antithyroidal drug or inorganic iodine. We classified the 20 patients on the basis of atrial fibrillation reversion into two groups, one with reversion (group I) and the other without reversion (group II). In all 12 patients in group I, T4 and T3 decreased to hypothyroid levels in 3.2 +/- 1.3 months, and one month later all patients had their sinus rhythm restored while T4 and T3 remained below normal (2.6 +/- 1.1 micrograms/dl and 77.9 +/- 34.4 ng/dl, respectively). Although T4 and T3 also decreased within 3.5 +/- 1.8 months in all 8 patients in group II, one month later, atrial fibrillation persisted while T4 and T3 (10.4 +/- 5.3 micrograms/dl and 157.7 +/- 67.5 ng/dl, respectively) rose significantly compared to those in group I (P less than 0.001 and P less than 0.01, respectively). For reversion of atrial fibrillation it is important that the onset of hypothyroidism is rapidly induced by radioiodine and that hypothyroidism continues for at least one month.