Letterer-Siwe syndrome /let″ərər zē″və/ [Erich Letterer, German pathologist, b. 1895; Sture A. Siwe, Swedish physician, 1897–1966] , any of a group of acquired malignant neoplastic diseases of unknown origin, characterized by histiocytic elements. The syndrome, fatal when untreated, occurs in infancy. Anemia, hemorrhage, splenomegaly, lymphadenopathy, and localized tumefactions over bones are usually present.
