We report the case of a young female patient with chronic active Epstein-Barr virus infection (CAEBV) and severe hypersensitivity to mosquito bites (SHMB). She showed a marked increase of NK cell population in peripheral blood. The NK cell population was suggested to be infected with EBV, and to be oligoclonal by Southern blotting using an EBV genome terminal-repeat probe. The NK cells aberrantly expressed CD25, a high affinity receptor for IL-2, and showed an augmented in vitro proliferative response to IL-2. Moreover, they also showed enhanced expression of both Fas-ligand and Bcl-2, and resistance to in vitro Fas-induced apoptotic cell death (Fas-ACD). Taken together, these observations suggested that both the augmentation of proliferative response to IL-2 and the decrease in Fas-ACD may cause NK cell lineage granular lymphocyte proliferative disorder (NK-GLPD) in patients with CAEBV and SHMB.