Previous studies suggest a role of viral infection in the development of Hashimoto's thyroiditis (HT). Here we report a patient with HT in whom human parvovirus B19 (B19) DNA has been persistently detected in the thyroid regardless of the presence or absence of B19 DNA in peripheral blood mononuclear cells. In contrast to the DNA persistence, however, VP1 capsid protein was not detected in the thyroid by immunohistochemical studies. Thyroid specimens obtained by fine needle aspiration biopsy from two patients with HT and two with Graves' disease were negative for B19 DNA. Thus, whereas a causal link between B19 infection and HT remains to be determined, B19 DNA may persist in the thyroid and B19 infection may facilitate the intrathyroidal inflammatory process in HT patients.