Spontaneously resolving hyperthyroidism (SRH) is a transient form of hyperthyroidism characterized by a painless, nontender, normal-sized or slightly enlarged thyroid gland, elevated levels of thyroxine and triiodothyronine, a depressed thyroid radioactive iodine uptake (RAIU), spontaneous resolution in two to five months, and a focal or diffuse lymphocytic thyroiditis on biopsy. Since 1962, 62 episodes of SRH were found in 56 individuals. Twelve thyroid biopsy specimens showed diffuse or focal lymphocytic thyroiditis. Testing of viral antibodies to a wide range of viruses demonstrated only one patient with notable titer change of 18 tested. Resolution of the hyperthyroidism took two to five months. Four patients were treated inappropriately. Lymphocytic thyroiditis with spontaneously resolving hyperthyroidism appears to be a new syndrome that has dramatically increased in frequency in the past ten years. It is difficult to differentiate from common forms of hyperthyroidism unless RAIU is determined.