Langerhans cell histiocytosis (LCH), previously called histiocytosis X, refers to a spectrum of disease characterized by idiopathic proliferation of histiocytes producing focal or systemic manifestations. Causes and pathogenesis remain unclear. However, recent studies suggest abnormal immune regulation as an important factor. The three classic syndromes may have considerable clinical overlap: eosinophilic granuloma, in which the disease is limited to bone in patients usually 5-15 years old; Hand-Schüller-Christian disease, characterized by multifocal bone lesions and extraskeletal involvement of the reticuloendothelial system (RES) usually seen in children 1-5 years old; and Letterer-Siwe disease, in which there is disseminated involvement of the RES with a fulminant clinical course in children less than 2 years old. Osseous involvement is typically in the flat bones, with lesions of the skull, pelvis, and ribs accounting for more than half of all lesions. About 30% of lesions are in long bones. Radiographic appearance of osseous LCH depends on site of involvement and phase of the disease. Early lesions appear aggressive with poorly defined margins and lamellated periosteal reaction. Late lesions appear well defined and may show sclerotic margins and expanded remodeled appearance.