Pulmonary Langerhans cell histiocytosis (PLCH), also known as pulmonary eosinophilic granuloma (PEG), is a chronic progressive disorder characterized by a proliferation of Langerhans cells infiltrates which form multiple bilateral interstitial peribronchiolar nodules that frequently cavitate. It primarily affects adults between the ages of 20 and 40. It is more common in men although the prevalence is increasing in women. The pathogenesis is unknown although there is a near universal association with cigarette smoking. The radiological manifestations typically consist of a symmetric bilateral nodular or reticulonodular pattern that is diffuse throughout the upper and mid lung zones but spares the costophrenic angles. The lungs show multiple bilateral nodules with varying degrees of cavitation. Histologically the nodular infiltrates have a stellate border extending into the surrounding interstitium. The lesions are centered on bronchioles. Lesions are also seen along the pleura and interlobular septa. The cellular infiltrate extends into the adjacent alveolar septal interstitium in a stellate pattern. The Langerhans' cells appear uniform with moderate eosinophilic cytoplasm prominently grooved nuclear membranes. Organizing pneumonia may be seen at the edges of the nodular infiltrates. The Langerhans cells react with stains for S100 and CD1a.