Pulmonary histiocytosis X is an uncommon but important cause of pulmonary fibrosis and honeycombing in young adults. This article reviews the pathologic, clinical, radiographic, and high-resolution computed tomographic (HRCT) features of pulmonary histiocytosis X, focusing on differential diagnosis and disease progression. The main pathologic feature of pulmonary histiocytosis X is peribronchiolar inflammation, leading to fibrosis and cyst formation. Although the diagnosis of pulmonary histiocytosis X may be suspected on the basis of chest radiographic findings, the HRCT findings of predominantly upper lobe nodules and cysts are virtually pathognomonic of this disorder. As pulmonary histiocytosis X progresses, the nodules decrease in number, leaving multiple thin-walled cysts. HRCT can be useful in visually estimating the proportion of lung involved and is also valuable in distinguishing histiocytosis from other disorders that produce nodules or cysts. Chest HRCT helps confirm the pattern seen at radiography and is valuable in establishing the correct diagnosis.