The object of the ILO/UC International Classification of Pneumoconioses and the general instructions for its use in asbestos respiratory diseases have been set forth. The ILO 1979 classification has been used in four groups of asbestos exposed subjects : two groups were hospitalized and two groups were at work. All the subjects have been investigated by a hospitalized and two groups were at work. All the subjects have been investigated by a standardized questionnaire on occupational history, PA and lateral chest X-rays and, for most them, by ferruginous bodies counting either in sputum or in bronchoalveolar lavage. The most frequent radiological signs were parenchymal fibrosis, pleural thickening, pleural calcification and diaphragmatic straightness. The fibrosis and the diaphragmatic straightness were related to the length of the exposure. Up to 20-29 years since the onset of exposure, the frequency of radiological signs was : diaphragmatic straightness more frequent than fibrosis which was more frequent than pleural thickening, which was more frequent than calcification. But the frequency of only two signs, pleural straightness and fibrosis, appeared significant after 10 years of exposure. The value of these four radiological signs as biological indicators of asbestos exposure is discussed.