The following study was performed with both the informed consent of the patient and the approval of the responsible committee in our hospitals. The clinical features and laboratory data of the patient have been previously reported.5 Briefly, an 18-year-old woman had recurrent necrotizing papules on the face and oral mucosa for 8 years. Since she was 13 years old, she has also had intense skin reactions at mosquito bite sites. The skin reaction usually began with erythema and swelling at 12 to 24 hours after the mosquito bite and developed to bulla, hemorrhagic necrosis, and ulcer formation (Figure 1). In addition to these local cutaneous manifestations, she also had systemic symptoms such as high fever, lymphadenopathy, hepatosplenomegaly, and general malaise. Hematologically, the typical cell morphology of large granular lymphocytes with azurophilic granules in an abundant cytoplasm was observed in approximately half of the mononuclear cells in a smear of the patient's peripheral blood. Flow cytometric analysis of lymphocytes showed a marked increase (51.1%) in the CD56+, CD16+, CD3− populations, which was suggested to be NK cells. The pattern of serum antibody titers against EBV was compatible with chronic active EBV infection as follows: the anti-EB viral capsid antigen (VCA) IgG and anti-EB early antigen (EA) IgG levels were markedly increased (×2560 and ×320, respectively). Polymerase chain reaction (PCR) analysis after sorting lymphocytes demonstrated that EBV DNA existed in NK cells but not in CD4+ T, CD8+ T, or CD19+ B cells. Southern blot analysis using EBV-terminal repeat probe demonstrated that EBV-positive NK cells oligoclonally proliferated. In situ hybridization using EBV-encoded small nuclear RNA1 (EBER1) probe demonstrated that more than 90% of NK cells were positive for EBV. Reverse transcription (RT)-PCR analysis demonstrated no expression of EBV lytic-cycle genes and type II latency expression pattern of EBV latent gene (ie, EBER1+, EBV nuclear antigen 1 [EBNA1]+, EBNA2−, latent membrane protein 1 [LMP1]+, LMP2A−, or LMP2B−).