A male patient, aged 23 years, with no notable previous history, was admitted to hospital because of fever, splenomegaly, and pancytopenia. The bone marrow was infiltrated by mature looking histiocytes displaying erythrophagocytosis. Other findings included polyclonal hypergammaglobulinemia, elevated transaminases, hypofibrinogenemia, and hypertriglyceridemia. A diagnosis of virus-associated hemophagocytic syndrome was made and virology studies attributed it to herpes simplex virus type 1. Therapy with acyclovir, high dose i.v. immunoglobulins, and vinblastine was unsuccessful. With the addition of prednisone, a remission, with disappearance of histiocytic marrow involvement, was obtained but was of short duration, and the outcome was fatal.