Pulmonary Langerhans cell Histiocytosis is a rare granulomatous disease affecting both sexes, with greater incidence in the second and third decades of life; smoking appears to be the most important risk factor. Its etiology is unknown, although there are data indicating an uncontrolled immune response as possible cause, sustained by the Langerhans cells, antigen presenting cells for T lymphocytes, and their accumulation in the distal bronchioles; these cells express on their surface the CD1a and CD1c antigens, and the B7 molecule, essential for activating quiescent T lymphocytes. In its evolution the granuloma is characterized by the progressive reduction in the LC number, with the increase of fibrosis, surrounding and destroying the bronchiolar lumen; the remaining of the lumen, or the traction exerted by fibrous tissue on the adjacent alveolar spaces leads to the cyst development. Vascular involvement occurs frequently, and may explain the onset of pulmonary hypertension in advanced cases of the disease. The disease may be asymptomatic, or it may present with aspecific respiratory signs and symptoms, and has characteristic radiological findings, being included in the group of cysticaerial parenchymal alterations. The diagnosis could be suggested by the finding of a number of LC in BAL greater than 5%. Different therapies have been proposed, but it seems that the most important measure is smoking cessation.