Rationale: From 1921-1990 vermiculite ore from Libby, Montana was shipped worldwide for commercial and residential use. A 1980 study of a manufacturing facility utilizing Libby vermiculite was the first to demonstrate a small but significant prevalence of pleural chest radiographic changes associated with amphibole fibers contained in the ore. Objective: This follow-up study of the original cohort evaluated the extent of radiographic changes and cumulative fiber exposure 25 years after cessation of exposure. Methods: From the original 513 cohort of workers, 431 (84%) were living and available for participation and exposure reconstruction. Of these, 280 (65%) completed both chest radiographs and interviews. Primary outcomes were pleural and/or interstitial changes. Results: Pleural and interstitial changes were demonstrated in 80 (28.7%) and 8 ( 2.9%), participants respectively. Of those participants with low lifetime cumulative fiber exposure (CFE) of <2.21 fiber/cc-years, 42 (20%) had pleural changes. A significant (p<0.001) exposure response relationship of pleural changes with CFE was demonstrated, ranging from 7.1% to 54.3% from the lowest to highest exposure quartile. Removal of individuals with commercial asbestos exposure did not alter this trend. Conclusions: This study indicates exposure within an industrial process to Libby vermiculite ore is associated with pleural thickening at low lifetime CFE levels. The propensity of the Libby amphibole fibers to dramatically increase the prevalence of pleural changes 25 years after cessation of exposure at low CFE levels is a concern in view of the wide national distribution of this ore for commercial and residential use.