Patients with Down's syndrome have an increased prevalence of autoimmune disorders that affect both the endocrine and non-endocrine organs. The most common thyroid abnormality in Down's syndrome is subclinical hypothyroidism (12.5~32.5%). The occurrence of Down's syndrome in conjunction with hyperthyroidism is rare (0.6~2.5%). A 35-year old female was transferred to our hospital because of hypotension and mental change. She had suffered from a poor oral intake and general weakness for the previous 1 week. She had been admitted local hospital and was diagnosed as hyperthyroidism. On the third day after admission, she lost consciousness and was then transferred to University Hospital. Physical examination revealed hypotension (76/39 mmHg), sinus tachycardia (111/min) and tachypnea (28/min). The upward-outward slant of the palpebral fissures, epicanthal folds, low-set ears, short stature and clinodactyly were all identified. The thyroid gland was not enlarged and there was no evidence of ophthalmopathy. The serum free T4 concentration was 3.32 ng/dL, the T3 level was 212 ng/dL and the TSH level was 0.01 uIU/mL. She was positive for TBII. Abdominal computed tomography showed ascites and pneumoperitoneum. Primary closure was done on the duodenal ulcer perforation site. She was treated with transrectal propylthiouracil and intravenous esmolol. Chromosomal analysis revealed 47XX and 21 trisomy. She was finally diagnosed as Down's syndrome, Graves' disease and duodenal ulcer perforation. Her hyperthyroidism was controlled with PTU 100 mg after discharge.