Epstein-Barr virus (EBV) DNA load in the blood increases in posttransplant lymphoproliferative disorders and chronic active EBV infection. In this report, we analyzed the EBV DNA load in the peripheral blood mononuclear cells (PBMCs) and plasma of patients with hydroa vacciniforme (HV) and/or hypersensitivity to mosquito bites (HMB) to understand the clinical significance of EBV DNA load. All 30 patients showed high DNA loads in the PBMCs over the cut-off level. Of 16 plasma samples, extremely high in two samples obtained from patients with hemophagocytic lymphohistiocytosis (HLH). The amount of cell-free DNA in plasma was correlated to the serum levels of lactate dehydrogenase and inversely correlated to platelet counts. These results indicate that the EBV DNA load in PBMCs can provide one of the diagnostic indicators for HV and HMB and marked elevation of cell-free EBV DNA in plasma might be related to cytolysis such as that observed in HLH.