The fundamental ultrastructure of lipid storage in the xanthoma cells of various xanthomatous diseases, including familial hyperlipoproteinemia type IIa, III, and V, cerebrotendinous xanthomatosis, Wolman's disease, Tangier disease, Hand-Schüller-Christian disease, and normolipidemic cutaneous xanthomatosis, revealed lipid vacuoles, either membrane-bound or with no single unit membrane, cholesterol crystals, multivesicular or multilocular lipid bodies, myelin-like bodies, and ceroid granules (residual bodies). According to the presence or absence of such a single unit membrane and enzyme cytochemical demonstration of acid phosphatase activity, these lipid storage inclusions were largely classified into lysosomal and non-lysosomal ones; the former included membrane-bound lipid vacuoles, cholesterol crystals, multivesicular or multilocular lipid bodies, myelin-like bodies, and ceroid granules and the latter was lipid vacuoles with no limiting membrane. The ultrastructural relationship on formation of these lysosomal and non-lysosomal lipid storage inclusions and pathogenesis of the lipid storage in the xanthoma cells of the disorders were presented. As for the origin of the xanthoma cells, the majority of them were considered to be derived from macrophages in many of the disease; however, transformation of fibroblasts into xanthoma cells was confirmed in xanthomatous diseases, such as Hand-Schüller-Christian disease.