Bronchiolitis obliterans is a pathologically defined syndrome of bronchiolar obstruction by organizing exudate, often associated with cholesterol and interstitial pneumonia.Among 52 cases with roentgenograms available for review, few were associated with specific etiologic factors. Several patients had co-existent systemic disease.Although occasionally a localized process amenable to surgery, bronchiolitis obliterans was usually diffuse, causing pulmonary insufficiency and death in some, but resolving in others.Administration of steroids was usually associated with clinical improvement.Roentgenologic patterns included: nodular densities (micronodular, discrete, confluent and lineonodular); alveolar opacities (diffuse, multiple irregular or bibasilar); and hyperinflation.Early recognition of this syndrome may permit treatment before irreversible changes develop.