To define the pulmonary functional impairment associated with pleural asbestos signs (PAS), we compared 738 men with only circumscribed (plaques) or diffuse pleural thickening on chest roentgenograms but no irregular opacities by ILO pneumoconiosis criteria (1980) with 738 age-matched asbestos-exposed men without any roentgenographic signs and with 228 men unexposed to asbestos. All men were white. Spirometry and total thoracic gas volumes (TGV) were measured and expressed as percentage of predicted of white Michigan men who have been modeled for spirometric values thereby adjusting for height, age, and in current and ex-smokers for duration of smoking. Asbestos-exposed men who never smoked had reduced FEF75-85 (p less than 0.01) and increased TGV (p less than .0001) as compared with unexposed men. The 155 men with PAS who had never smoked had reduced flows (p less than .0001), FVC (p less than 0.0056), and TGV (p less than .0001) when compared with 155 age-matched asbestos-exposed men. The 325 asbestos-exposed current smokers with normal chest roentgenograms compared with unexposed smokers had reduced expiratory airflows (p less than 0.0001), reduced FEV1 (p less than 0.004), and increased TGV (p less than 0.0001). The 325 current smokers with PAS had additional air trapping that further reduced vital capacity. Thus, PAS were associated with significant pulmonary dysfunction in men who never smoked, and current and ex-smokers had additional dysfunction even after adjustment for duration of smoking.