Histiocytosis X is a term coined by Lichtenstein in 1953 to identify a disease pattern of the reticuloendothelial system, and Histiocytosis X presents a wide spectrum of clinical pictures in which the underlying common denominator is the development of granulomatous lesions with histocytic proliferation. The clinical expression appears to be as reactive phenomenon, the triggering mechanism for which is unknown, and may range from an isolated, slow growing lesion, particularly in the medullary cavity of bone, to aggressive, widely disseminated disease with fatal outcome. Conversion from a localized to a systemic form does occur. Seperate diagnostic terms : Eosinophilic granuloma, Hand-schüller-Christian disease and Letterer-Siwe disease, were suggested for the different forms of the disease. We have experienced a case of eosinophilic granuloma in year old male and a case of Letterer Siwe disease in year old male and reported with literature.