Epstein-Barr virus (EBV) is a common virus that latently infects most adults and has a tropism to B lymphocytes. In 1988, two cases of EBV infection were reported to be eradicated by hematopoietic stem cell transplantation from an EBV-negative donor. However, the dynamics of EBV after cord blood transplantation (CBT), namely, the kinetics of anti-EBV antibodies, the incidence of negative/adverse seroconversion (from positive to negative), and the clinical course of re-infection (second primary infection) by EBV, have not yet been characterized in detail. Therefore, we performed a nationwide survey that focused on the dynamics of EBV after CBT 1 year or later after CBT. Negative seroconversion occurred in 23% of previously EBV-infected patients. The incidence of late-onset EBV-associated events was 1.9% (13/674) : 5 infectious mononucleosis, 2 hemophagocytic lymphohistiocytosis (HLH), and 6 remaining typical lymphoproliferative disease. HLH occurred in newly infected patients (primary or second primary) and also in those with reactivation and was fatal. The annual monitoring of anti-EBV antibody titers may facilitate the early detection of these late-onset EBV-associated events and treatment initiation before disease progression.