Langerhans cell histiocytosis (LCH) is a group of disorders characterised by monoclonal proliferation of histiocytic cells (Langerhans cells) producing tumour-like masses in multiple organs including the bone, skin, lymph nodes and central nervous system. In contrast to multiorgan LCH, pulmonary LCH (PLCH) usually involves only a single organ and presents as an infiltrative lung disease. PLCH is strongly correlated with smoking and presumably reflects reactive Langerhans cell proliferation triggered by some inhaled agent.1 In early cellular PLCH, Langerhans cells aggregate in multiple small bronchiolocentric granulomas which may further cavitate to form inflammatory thick-walled cysts, usually predominating in the upper lobes. With disease progression, PLCH may evolve towards irreversible lung destruction by cicatricial fibrotic thin-walled cysts, respiratory insufficiency and death or …