To assess the clinical significance of asbestos-induced pleural fibrosis, we evaluated the relationship between radiographic evidence of pleural fibrosis and spirometric values in 1,211 sheet metal workers. Of those with pleural fibrosis (n = 334), 78% had circumscribed plaques and 22% had diffuse pleural thickening involving the costophrenic angle. Factors that were found to be associated with the presence and type of pleural fibrosis included increased age (p less than 0.001), more years in the trade (p less than 0.0001), more years since first exposure to asbestos (p less than 0.0001), more pack-years of cigarette smoking (p less than 0.01), and the presence and degree of interstitial fibrosis (p less than 0.0001). After controlling for these potential confounders (age, years in the trade, latency, pack-years of smoking, and ILO profusion category), linear multivariate regression models demonstrated that both circumscribed plaques (p = 0.007) and diffuse pleural thickening (p = 0.008) were independently associated with decrements in FVC but not with decrements in the FEV1/FVC ratio. Furthermore, our data indicate that the effect of diffuse pleural thickening on decrements in FVC is approximately twice as great as that seen with circumscribed pleural plaques. We conclude that the presence and type of pleural fibrosis among asbestos-exposed workers is independently associated with a pattern of spirometry that is suggestive of an underlying restrictive defect in lung function.