Histiocytosis-X is a term used to define three diseases with similar morphologic characteristics : Letterer-Siwe diseae, Hand-Schuller-Christian disease and Eosinophilic granuloma. In general, they differ in terms of their age of onset, severity of clinical course and site of involvement. Eosinophilic granuloma typically is seen in young adults. Eosinophilic granuloma is diagnosed in the presense of diffuse pulmonary infiltrate, bony involvement. However, the Pulmonary radiologic findings of eosinophilic granuloma are variable accordinary to stage of disease. therefore pathologic diagnosis of involving site is essential for confirmative diagnosis of eosinophilic grananuloma. Pathologically. the three disease are characterized by granulomatous infiltration of alveolar septa and bronchial walls and often involvement of bone. The hallmark of this disease is proliferation of the Langerhans' cell. The identifying feature is the X-body or Birbeck granule that is present in Langerhans' cells and histiocytic cells found in the lung of EG patient. We report a case of bilateral, recurrent and spontaneous pneumothoraces in a 21 year old man with pulmonary histiocytosis-X which is confined by eosinophilc granuloma in bone marrow biopsy and ultrastructural examination in cells obtained from BAL.