Abstract FEL is an autosomal recessive, lethal disease of early childhood characterized by hepatosplenomegaly, fever, and multisystem lymphohistiocytic infiltrates. The etiology and pathogenesis of this disorder remain uncertain. However, evidence suggests that the disease may be due to an inherited defect in immunoregulation that predisposes to an uncontrolled proliferation of activated histiocytes in response to a stimulus such as viral infection. Although clinical remission can often be achieved with systemic VP-16 and aggressive CNS therapy, the disease usually becomes refractory to treatment with a fatal outcome. Bone marrow transplantation may prove to be an effective treatment for this tragic disease.