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Prolongation of the P-R interval has been reported in uncomplicated hyperthyroidism, 1-5 and some of the patients show, in addition, large and tall P waves in leads II, III, and aV F . Transient complete heart block has been described also in hyperthyroid patients, particularly in association with acute inflammatory diseases. 6-11 Electrocardiographic abnormalities usually disappear when the patients become euthyroid. We are reporting the cases of two hyperthyroid patients without signs of heart disease who had first-degree heart block with tall and large P waves; in one of these patients, a left bundle-branch block and a transient complete heart block with Stokes-Adams episodes also occurred without clear-cut evidence of acute inflammatory disease. Both patients eventually showed a normal electrocardiographic pattern when they became euthyroid. PATIENT SUMMARIES Patient 1. —A 19-year-old woman was admitted with symptoms of weakness, fatigue, fine tremors, and palpitations. She had been found to be hyperthyroid one