The chest roentgenograms of 18 patients with pulmonary histiocytosis X were reviewed. Correlation of roentgenographic and histologic changes in 6 patients disclosed a spectrum of changes reflecting the evolutionary character of the pulmonary histiocytotic lesion.This spectrum is arbitrarily divided into 3 categories. The first two are based on the site of maximal infiltration by the proliferating histiocyte, either alveolar or interstitial. The third represents the reparative phase, with fibrosis as the dominant feature.[See figure in the pdf file]In addition to the 6 case reports, 4 cases of mediastinal and hilar lymphadenopathy and a single case of pneumomediastinum are added to the literature.