Silent thyroiditis (ST) is the second major cause of thyrotoxicosis, following Graves' disease (GD) in Japan. Thyrotoxicosis develops differently in these two disorders: GD stimulates TSH receptors through antibody binding, while ST destroys follicles, probably through an autoimmune mechanism. Distinguishing between ST and GD depends mainly on serum TSH receptor antibody titer and radioactive iodine uptake. In our long-term observation of ST patients, ST recurred with a high incidence (65%, 35/54). Some ST patients developed GD later, and ST developed in patients with a history of GD. Thus, the differential diagnosis between these two diseases is very important, even when a patient has a history of either ST or GD. A simpler diagnostic test specific to ST would be of great use.