The clinicopathological and immunohistochemical features of four patients with systemic multicentric reticulohistiocytosis (MR) were compared with five cases of solitary and one case of multiple reticulohistiocytoma (RH), which were confined to the skin only. The MR cases mostly affected the limbs of older women, while RH affected young male adults without preference to site. Characteristically, both entities consisted of oncocytic mononuclear histiocytes (with granular eosinophilic cytoplasm similar to oncocytic thyroid cells) and multinucleated histiocytes with a ground-glass appearance, which appeared to be much larger (> 200 microns) and bizarre in cases of RH compared with cases of MR (50-100 microns). In RH a variable number of vacuolated, spindle-shaped, and xanthomatized mononuclear histiocytes were also present. Immunohistochemical profiles showed positivity of mononuclear histiocytes with HHF35, factor XIIIa, and LN3 (HLA-DR), with a variable number of multinucleated histiocytes in RH showing binding with peanut agglutinin. In mono- and multinucleated histiocytes in both entities macrophage markers KP1 (CD68), KiM1P, HAM56, lysozyme, and alpha 1-antitrypsin were positive. However, macrophage markers MAC387 (L1 antigen) and Leu-M1 (CD15) were negative. Vimentin was universally positive in both conditions, with all other markers (S100, desmin, smooth muscle-specific actin, and QBEnd 10 [CD34]) negative. This study shows that histology supplemented by immunocytochemistry delineates MR from RH and immunohistochemical profiles indicate a cell lineage relationship between RH and adult xanthogranuloma.