Previously we described a lesion of the small airways that appears related to mineral dust exposure and is found in asbestos and nonasbestos dust-exposed populations. To determine the usefulness of this lesion as a marker for mineral dust exposure, and to determine whether it produces functional consequences, we examined a group of 53 workers who had been either hard rock miners or in the asbestos, construction, and shipyard industries. The specific lesion (mineral dust airways disease (MDAD] consists of marked fibrosis and pigmentation of the respiratory bronchioles and was found in 13 of 53 workers with dust exposure, but only in 1 of 121 without dust exposure. Compared with age and smoking-matched dust-exposed control subjects, patients with this lesion had significant abnormalities of forced expiratory volume in one second (FEV1), forced expiratory flow during the middle half of the forced vital capacity (FEF25-75), vital capacity (VC), and nitrogen washout. In addition to fibrosis in the walls of respiratory bronchioles, these patients also had significant increases of fibrosis in the walls of membranous bronchioles, indicating that changes in the small airways are widespread in this subset of workers. We conclude that markedly abnormal small airways are present in some workers with mineral dust exposure; pathologic observation of this lesion is a good indicator of dust exposure, and its presence is associated with abnormalities of air flow greater than those induced by smoking alone. The presence of this lesion in only a portion of dust-exposed workers may account for contradictory results in past studies that attempted to demonstrate air flow abnormalities associated with mineral dust exposure.