The lung function tests on 88 asbestos workers known to have an asbestos-related abnormality on the chest radiography have been analysed with respect to pleural disease. The degree of intrapulmonary and pleural disease were assessed by reading posteroanterior and both oblique films according to a modified ILO U/C Classification (1971). The results show a highly significant decrease in lung function with increasing pleural abnormality. Analyses suggest that these changes cannot be wholly accounted for by confounding variables such as airways obstruction and/or pulmonary fibrosis. We conclude that pleural thickening alone, particularly when severe, contributes to the decreased vital capacity and to any resultant disability.