A study was made between 1966 and 1988 of lung function and asbestos (1332214) related circumscribed plaques in 1764 workers. Subjects were surveyed at two shipyards, three paper mills and an asbestos facility. Cases included 218 (12.3%) workers with plaques only and no diffuse thickening or asbestosis and 176 (10.0%) workers with pleural plaques and asbestosis. Two comparison groups were used: 100 unexposed males at least 40 years old without prior asbestos exposure or discernible lung disease; and 154 workers with normal chest x-rays, matched for age and years of exposure (15 or more) with plaque only cases. Chronic obstructive lung disease (COLD) was diagnosed based on the ratio of forced expiratory volume in 1 second (FEV1) to forced vital capacity (FVC). COLD was observed in 16.2%, 9.6% and 14.2% of those with normal x-rays, plaques only and plaques with asbestosis, respectively. Years since first exposure were nearly identical in all exposed groups. Exposed comparisons had slightly lower lung function values than those of unexposed comparisons. Slightly higher values of FEV1, FVC and single breath diffusing capacity (DL) were noted in the plaque only group relative to all other groups. There were no differences in lung function values associated with plaque size or calcification (observed in 80 workers). Progressing asbestosis produced precipitous declines in lung function parameters, with DL decreasing the most. Smokers and exsmokers in all groups had significant reductions in FEV1 and DL relative to values for all nonsmokers. Diffuse thickening, noted in 62 cases, produced significantly decreased lung function, particularly FVC. This was related to extent and thickness of lesions. The authors conclude that circumscribed plaques alone do not affect lung function with respect to volume, flow and gas exchange, but diffuse thickening can cause significant pulmonary insufficiency in the absence of asbestosis.