ABSTRACT
Histiocytosis X is an uncommon multisystemic disease of unknown etiology which is characterized by the accumulation of histiocytes in various tissues, and variable clinical course. In patients with histiocytosis X, orbital involvement is common and has a distinct prediction for the superotemporal orbital bone. Orbital involvement is almost invariably associated with a osteolytic lesion of the orbital wall. Disease confined to orbital soft tissues without bone involvement is rare. We report the clinicopathologic features of an unusual form of histiocytosis X in a 2-year-old female who showed a rapidly growing lower eyelid mass. No other ocular or extraocular involvement was detected. The mass was excised and diagnosis was confirmed by hematoxylin eosin staining, which showed large histiocytes and a number of eosinophils, plasma cells and lymphocytes. In addition, the histiocytes were immunohistochemically stain for s-100 protein.