We report a rare case of Basedow's disease associated with high output heart failure and angina pectoris over the age of 80 years. An 85-year-old woman was admitted with palpitation, finger tremor, hyperidrosis and weight loss. Basedow's disease was diagnosed by physical (diffuse goiter) and laboratory (free T3 19.4 pg/ml, free T4 > 8.0 ng/dl, TSH < 0.1 microU/ml, TRAb positive, 123I uptake high) findings and was treated with methimazole. Chest oppression and dyspnea on exertion with negative T wave, cardiomegaly and pulmonary congestion appeared after methimazole. Cardiac catheterization showed a high cardiac output (CI 5.01/min/m2, PCW 26 mmHg, PA 57/26 mmHg, RA 15 mmHg) and a significant coronary stenosis (LAD [symbol: see text] 99%). High output heart failure and angina pectoris responded to treatment. They subsequently worsened, because she stopped taking methimazole for a month and serum levels of thyroid hormones increased again. After retreatment with methimazole, serum levels of thyroid hormones decreased to within normal limits, and high output heart failure and angina pectoris also improved.