A 51-year-old woman had symptoms of thyrotoxicosis which disappeared spontaneously within two months. She was diagnosed as a case of silent thyroiditis on the basis of both the clinical course and the laboratory data such as low uptake of radioactive iodine and technesium. She also had petechiae in her arms which were diagnosed as an idiopathic thrombocytopenic purpura (I.T.P.). This case would seem to expand the spectrum of the coexistence of autoimmune thyroid diseases and I.T.P. which is believed to be an autoimmune disease.