Small-airway lesions were identified in histologic sections from the lungs of seven workers with histories of exposure to nonasbestos dusts. The lesions consisted of deposits of fibrous tissue, often accompanied by pigment, in the walls of membranous and respiratory bronchioles and alveolar ducts. Comparison with a matched population of persons with no dust exposure revealed that the changes in the respiratory bronchioles and alveolar ducts were morphologically distinctive and could be used to diagnose the lesions. Structurally, these lesions were very similar to those described in the airways of asbestos workers, although comparison with the authors' previous results indicated that the number of severely affected airways was less in the nonasbestos dust group. Mineralogic analysis indicated that these abnormalities were produced by a variety of different dusts including silica, iron oxide, and aluminum oxide, and that occult asbestos exposure, although possible in three of the cases, was most likely not a primary cause of disease. The authors suggest that this lesion, which they call mineral dust airways disease, is a nonspecific reaction of the small airways to inorganic particulates. The presence of such changes casts considerable doubt on the theory that small-airway abnormalities in asbestos workers are the earliest form of asbestosis.