OBJECTIVES: To analyse quantitatively the relations of spirometric lung function (forced vital capacity (FVC)) to radiographic interstitial pulmonary fibrosis (assessed by the International Labour Organisation (ILO) profusion score of small irregular opacities) in two large workforces exposed to different intensities of asbestos. These analyses consider the question whether a similar profusion score n differently exposed workers is associated with a similar effect on lung function. METHODS: Surveys of two workforces, insulators (n = 2611) and sheet metal workers (n = 1245), by the same investigators allowed comparison of the effects of the two levels of exposure to asbestos. The two groups were of similar age and had similar percentages of non-smokers and smokers. All radiographs were read by the same expert reader. RESULTS: Consistent with their less continuous and less intense exposure to asbestos, metal workers had: (a) far less frequent radiographic asbestosis (profusion score > or = 1/0, 17.5% v 59.6% for insulators): (b) less severe radiographic asbestosis (only 1.1% had scores > or = 2/1 v 13.3% of insulators); (c) a similar slope to that seen in insulators for the relation between FVC and profusion score when pleural thickening was absent; (d) less frequent pleural fibrosis (36% v 75%); and (e) less frequent restrictive impairment (23% v 33%). In both insulators and metal workers, lung function was below normal even when lung fields were normal, FVG fell with increasing profusion, it was lower in smokers and in those with pleural thickening at comparable profusion scores, and these was no difference in FVC between scores 0/1 and 1/0. CONCLUSION: The decrease in FVC with increasing profusion score in both workforces as well as the similar slopes for the relation between FVG and profusion score and the similar FVG at similar scores in the absence of pleural thickening confirm the ILO profusion score as an acceptable assessment of pulmonary fibrosis.