Juvenile xanthogranuloma (JXG) is considered to represent a lesion originating from histiocytes. Three cases of deeply located JXG and one case of cutaneous JXG were studied. One case with extensive mesenteric involvement presented with hypercalcemia and one case with liver involvement had hypergammaglobulinemia. Immunohistochemistry, electron microscopy, karyotyping, and DNA flow cytometry were used to determine the phenotype of the cells involved and to find further clues as to the histogenesis of these lesions. Immunohistochemically, all lesions studied expressed the CD1a antigen but showed no labeling for S-100 protein. The cells did not contain Birbeck granules. From these data it is suggested that the cells involved are of indeterminate dermal histiocyte lineage and that occurrence of deep located lesions of JXG may be due to migration of CD1 a-positive histiocytes.