Pulmonary function was measured in 79 men with diaphragmatic pleural plaques (DPP) as the only abnormality characteristic of asbestos disease on chest radiographs. They were selected from 4572 construction and shipyard workers exposed to asbestos. Abnormalities of pulmonary function in 21 non-smokers and 43 current smokers were compared with referent values adjusted for height, age, and duration of cigarette smoking. In the non-smokers, flows (FEV1) FEF75-85 and FEV1/FVC) were reduced and TGV and RV/TGV were raised. Current smokers had similar significant reductions. Thus by contrast with some current opinion that plaques are "an index only of past asbestos exposure," workers with plaques, even limited to the diaphragm, have functional impairment typical of pulmonary asbestosis. This suggests that they have pulmonary asbestosis, which is below the threshold of radiographic recognition.