Pleural fibrosis due to asbestos exposure was fully appreciated considerably later than pulmonary interstitial fibrosis due to similar exposure. This is well exemplified by the fact that pleura face on was included in the International Labour Office's International Classification of Radiographs of Pneumoconioses only in the last revision of the Classification. The functional relevance of pleural fibrosis, in particular circumscribed pleural fibrosis, has remained controversial. Since pleural fibrosis can occur at various sites (diaphragmatic plaques, chest wall in profile and face on, mediastinal) and can be of different thickness and extent, a comprehensive integrative assessment of pleural fibrosis was undertaken in order to permit a quantitative study of relationships between pleural fibrosis and pulmonary function. This approach was used for chest X-ray films of 1,584 asbestos insulation workers examined (1981-1983); 1,185 (75%) had pleural fibrosis. The distribution pattern of the integrative pleural index was found to be different in the subgroup with circumscribed (n = 975) from that with diffuse (n = 197) pleural fibrosis, with a higher profusion of high INDEX values in the latter. Stepwise regression analysis indicated that there was a significant inverse relationship between forced vital capacity (FVC) and the integrative index of pleural fibrosis in the subgroup with circumscribed pleural fibrosis. In the subgroup with diffuse pleural fibrosis, the obliteration of costophrenic angle(s), even with pleural fibrosis of limited extent, resulted in marked decrement in FVC% predicted; higher values of INDEX did not result in additional significant reductions of FVC. In those with both parenchymal and pleural abnormalities (n = 862) the pleural index was found to make a significant contribution, independent of that of parenchymal abnormalities, to decrements of FVC. Since pleural fibrosis has gradually become the predominant radiologically detectable abnormality in asbestos exposed workers, establishing its quantifiable functional relevance is useful.