A clinical variant of Langerhans cell histiocytosis characterized by unifocal involvement of a bone (most often), skin, or lung. Patients are usually older children or adults usually presenting with a lytic bone lesion. The etiology is unknown. Morphologically, eosinophilic granuloma is characterized by the presence of Langerhans cells in a characteristic milieu which includes histiocytes, eosinophils, neutrophils, and small, mature lymphocytes.