Assessment was undertaken of 165 x-ray photographs of the thorax originating from 165 men with varying degrees of exposure to asbestos: 37 were workers in an eternite factory, 65 insulation workers, 42 were plumbers and 21 were patients from the Lung Clinic in Bispebjerg Hospital. Assessment of the x-rays was carried out according to the directives established by the International Workers' Organisation (ILO) by three experienced specialists who had no knowledge of the individual person's exposure to asbestos or smoking habits. In order to investigate not only inter- but also intra observer variation, the three medical specialists described the material on two occasions with an interval of several days. Analysis of the descriptions revealed that there was considerable inter- and intra-observer variation in assessment of the changes caused by asbestos. Description of the parenchymatous changes was associated with slightly greater variation than description of the pleural changes. A partial analysis of smokers who had not had occupational exposure to asbestos showed a not inconsiderable occurrence (5-33%) of parenchymatous changes which could not be differentiated from changes caused by asbestos. It is concluded, that the diagnosis of pulmonary disease caused by asbestos should not be based on radiographic examination alone.