Langerhans' cell granulomatosis, formerly known as histiocytosis X include several granulomatosis of unknown aetiology. The original pathological finding is a Langerhans' cell granuloma. Recent progress has improved our understanding of the pathogenesis of this group of diseases and facilitated the diagnosis. We describe here the cell populations involved, the general pathology of Langerhans' cells, proposed pathogenesis, and the relationship between Langerhans' cell pulmonary granulomatosis and other localizations of Langerhans' cell granulomatosis. Finally, the clinical manifestations, elements of diagnosis and changing attitudes of current therapy are discussed.