We studied the morphologic, antigenic, and enzymatic characteristics of the histiocytes in an isolated extranodal case of sinus histiocytosis with massive lymphadenopathy (SHML) involving the CNS. To our knowledge, this represents the first immunohistochemically documented case of CNS SHML. The histiocytes exhibited the S-100(+), CD11c(+), alpha-1-antichymotrypsin(+) immunophenotype, which suggests that the histiocytes of SHML coexpress phenotypic characteristics of histiocytes of the mononuclear phagocytic system, and histiocytes of the interdigitating reticulum cell and Langerhan cell lineages. To examine the argument that the histiocytes in SHML may represent ordinary tissue macrophages of granulomatous inflammation, we compared the immunophenotypic characteristics of the histiocytes found in SHML to those found in xanthogranulomatous pyelonephritis (XP). We found that the histiocytes of XP have immunophenotypic characteristics of histiocytes belonging to the mononuclear phagocytic system lineage. The present study demonstrates that the histiocytes of SHML are distinct from the histiocytes of another xanthogranulomatous disorder, supporting the concept that SHML is a distinct clinicopathologic entity.