Histiocytosis X is a proliferative disorder of Langerhans cells. It is characterized by granuloma formation in the lung tissue to accumulation of abnormal histiocytes (histiocytes cells or Langerhans cells). The propose of this study is to report the cytological, inmunocytochemical and ultrastructural features of this entity as they appear in BAL sediments which may lead to precise diagnosis of pulmonary histiocytosis X.