Respiratory bronchiolitis is a mild inflammatory reaction commonly noted in asymptomatic cigarette smokers. We reviewed 18 cases of respiratory bronchiolitis-associated interstitial lung disease (RB/ILD), which had been diagnosed on the basis of clinical evaluation and open-lung biopsy. All patients were cigarette smokers. The sex distribution of the patients was approximately equal, and their mean age was 36 years. Chest roentgenograms showed reticular or reticulonodular infiltrates in 72% of the patients. Histologically, inflammation of the respiratory bronchioles, filling of the bronchiolar lumens and surrounding alveoli with finely pigmented macrophages, associated interstitial inflammation, and mild fibrosis were noted. In most patients, respiratory improvement ensued when they stopped smoking. Because of histologic similarities to desquamative interstitial pneumonia (DIP), the 18 cases of RB/ILD were compared with 36 cases of DIP. DIP tended to occur in older persons, caused more severe symptoms, displayed ground glass infiltrates on chest roentgenograms, was characterized by more severe interstitial disease on pulmonary function tests, and was often associated with progressive respiratory disease.