Three patients showing the rare association of Down syndrome and Graves' disease are reported. While two of the patients were asymptomatic, the third showed goiter, nervousness, weight loss, and tachycardia. In addition to the typical features of hyperthyroidism, this patient showed right heart failure and hypertransaminasemia, which disappeared with antithyroid treatment. Because Graves' disease is rare in children, and the clinical presentation was unusual in one of our patients, we report three patients with Graves' disease and Down syndrome, and emphasize the importance of periodic evaluation of thyroid function in children with Down syndrome not only to detect hypothyroidism.