Langerhans cell histiocytosis (LCH) is an abnormal proliferation of Langerhans cells, which are a form of dendritic cell. Pulmonary LCH in adults is a cigarette or marijuana smoke-induced disease because these forms of smoke act as dendritic cell attractants. Extensive evidence indicates that LCH is a neoplastic process, most commonly associated with BRAF V600E or MEK2K1 mutations. However, LCH confined to the lung typically results in scarring rather than metastases. LCH in the lung forms stellate cellular nodules early but a variety of fibrotic reactions in late disease. Smoking cessation is crucial to the treatment of pulmonary LCH, but data derived from patients with systemic or multisystem disease suggest that BRAF V600E or MEK2K1 inhibitors may work in the lung as well.