Three cases representing a familial disease not previously described are reported. The disease occurred in children under the age of 7 years and was characterized clinically by an acute but protracted course, with fever, anemia, leukopenia, thrombocytopenia, hepatosplenomegaly, pneumonitis and meningitis. Pathologically, there was extensive infiltration of many organs and tissues by lymphocytes and histiocytes with remarkable involvement of the central nervous system and neurohypophysis. The nature of this disease is uncertain, but a generalized cellular, metabolic or biochemical etiology is suggested. Significant differences from Letterer-Siwe disease and Chédiak-Higashi syndrome are pointed out.