We report the first case of hemophagocytic lymphohistiocytosis (HLH) induced by the monoclonal expansion of Epstein-Barr virus (EBV)-negative NK cells. Consanguinity of the patient's parents made it necessary to discard familial HLH in the patient and her sister with identical HLA markers and demonstrate that no cause other than the expansion of NK cells, which secrete high levels of gamma interferon, was inducing HLH in this patient. CASE REPORTA 17-month-old Ecuadorian girl was admitted to our hospital, Hospital Universitario Virgen de la Arrixaca, Murcia, Spain, in September 2005, because of a fever and cytopenias (leukocytes, 11.6 ϫ 10 9 /liter; hemoglobin, 7.9 g/dl; platelets, 65 ϫ 10 9 /liter) which had persisted for over 3 weeks, splenomegaly (10 cm), hepatomegaly (5 cm), multiple adenopathies, purpura, and edema. The father and mother were cousins and healthy, the patient's gestation and birth had been normal, but recurrent fever episodes were reported from 3 months of age. The girl was vaccinated normally. Biochemical analysis showed normal values for fibrinogen (232 mg/dl; range, 150 to 450 mg/dl) and elevated levels of ferritin (278 ng/ml; normal range, 15 to 150 ng/ml) and triglycerides (311 mg/dl; normal range, 50 to 200 mg/dl). Serology demonstrated anti-cytomegalovirus

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