A correlated light microscopical, histochemical and ultrastructural study‐was made on bone lesions repeatedly biopsied from three patients with Hand‐Scbüller‐Christian disease. The histiocytic cells proliferating in the early lesions had a markedly indented nucleus and contained In their cytoplasm varying numbers of Langerhans cell granules and round or irregular‐shaped lipid droplets. Histochemically, the lipid droplets were verified to be lipoprotein rich In fatty acids. Such histiocytic cells differ from highly phagocytic histiocytes or macrophages because of indistinct phagocytotlc activity, and are regarded as a metabolic type of histiocytes. The late lesions were mostly dominated by a xanthogranulomatous tissue chiefly comprised of xanthoma cells and collagen fibers. The xanthoma cells were usually filled with numerous lipid vacuoles which were histochemically proved to mainly contain cholesterol. Almost all the xanthoma cells were closely attached to collagen fibers and often had well‐developed rough endoplasmic reticula. Ultrastructurally, It is suggested that the xanthoma cells are originated from fibroblasts or flbrocytes.As an Important factor to induce proliferation and xanthoma cell transformation of fibroblasts in the late phase, the presence of such a lipoprotein produced in the histiocytic cells of the early phase is