36 cases of coexistent hyperthyroidism and idiopathic thrombocytopenic purpura (ITP) were reported. There were 33 cases of overt hyperthyroidism with ITP. After treatment with antithyroid drugs the platelet count returned to normal in 29 and somewhat increased in the remaining four cases. The other three cases had ITP accompanied by hyperthyroidism. In 2 cases the platelet count decreased when they had accompanying hyperthyroidism. When the thyroid function returned to normal after antithyroid treatment, the platelet count returned to normal in one and increased in another. The authors discussed the association between hyperthyroidism and ITP and treatment of coexistent hyperthyroidism and ITP. Probable mechanisms causing thrombocytopenia in hyperthyroidism were discussed also.