Down's syndrome (trisomy 21) has been frequently associated with thyroid disease, mainly subclinical hypothyroidism (12.5-32.5%). The occurrence of Downs syndrome in conjunction with hyperthyroidism is rare (0.6-2.5%). The mechanism that Down's syndrome was frequently associated with autoimmune thyroid disease is not clear, but T cell maturation defects and overexpression of chromosome 21 products in Down's syndrome have been suggested. A 19-year-old female was transferred because of generalized weakness. She was born to a 42-year-old mother, She had been suffered from heat intolerance, weight loss, palpitation, dyspnea on exertion and neck swelling and had intermittently taken some medication since her age 9. She had mental retardation (IQ 41) and underdeveloprnent. Exophthalmos, upward-outward slant of palpabral fissures, epicanthal folds, lowset ears, and large, protruding, fissured tongue were identified. Short fifth middle phalanges, clinodactyly and small-sized interventricular septal defect were also detected. Thyroid gland was diffusely enlarged four times the normal size, firm in consistency and had a bruit. Serum T concentration was 7.8ug/dL, T2 306ng/dL, and TSH 0.01ulU/mL. She was positive for thyroid autoantibodies (antimicosomal antibody 1,867 IU/mL, antithyroglobulin antibody 106 IU/mL, and TBII 79.6%). Twenty-four hours radioactive iodine uptake was 64%. Chromosomal analysis with T cell culture stimulated by phytohemagglutinin revealed 47XX, 21 trisomy. Pituitary hormones except TSH were fully stimulated by combined pituitary stimulation. She was finally diagnosed as Down's syndrome with Graves' disease and controlled with use of methimazole.