The authors describe an unusual case of a young man presenting with fever, asthenia, anorexia and jaundice, associated to hepatosplenomegaly, evolving rapidly to multiorganic failure. Final diagnosis revealed an aggressive NK cell leukemia/lymphoma associated to the Epstein-Barr virus (EBV). The diagnosis, suggested clinically and after bone marrow immunophenotyping, was confirmed by morphologic and immunohistochemical findings on the post-mortem hepatic and splenic biopsy .The tumor cells were positive for CD3 and cytotoxic molecules, TIA, granzyme B and perforin. The herein reported case is a rare clinical entity, only recently recognized and with a difficult early diagnosis. We emphasize the necessity to exclude a Natural Killer cell malignancy in cases with identical characteristics.