Epstein-Barr virus (EBV) is a herpes virus that mainly infects in B lymphocytes and occasionally reactivates lytically. Most individuals have been infected with EBV primarily in their childhood with no symptoms, and the virus persists latently for life. We have previously reported that EBV-infected B cells with thyrotropin receptor autoantibodies (TRAbs) on their surface [TRAb(+) EBV(+) cells] were present in the peripheral blood mononuclear cells (PBMCs) of healthy adult controls and patients with Graves' disease, and that TRAbs released in the culture medium of PBMCs containing TRAb(+) EBV(+) cells by EBV reactivation. EBV lytic reactivation induced the differentiation of host B cells into plasma cells and antibody production. Various autoantibodies have been detected during the acute phase of infectious mononucleosis (IM) that is the symptomatic primary infection of EBV. Therefore, the autoantibody production may be induced by the asymptomatic primary infection. In this study, we examined the presence of TRAb(+) cells, EBV(+) cells, and TRAb(+) EBV(+) cells in PBMCs from 29 healthy or subclinical children without Graves' disease and one cord blood that were divided into six age groups, and also measured plasma TRAb levels. The results obtained demonstrated that low levels of TRAb production occurred with EBV primary infection and lytic reactivation in children without symptoms of IM. Furthermore, the populations of TRAb(+) cells, EBV(+) cells, and TRAb(+) EBV(+) cells were small in the period of primary infection, but they potentially expand with repeated EBV lytic reactivation. This may partly explain why the onset of Graves' disease often occurs in young adults, but rarely in infancy.