Acute rheumatic fever and third-degree heart block developed in a 23-year-old man with hyperthyroidism. Although cardiac arrhythmias frequently complicate hyperthyroidism, third-degree heart block occurs only rarely in this disorder. Prolonged atrioventricular conduction has been noted in a significant number of hyperthyroid patients. Available information indicates that higher degrees of heart block may occur in hyperthyroidism when additional factors are present which can independently impair atrioventricular conduction.