258 workers in three asbestos factories were examined, by clinical, radiological and pulmonary function tests. Rales was found in 35 workers (14%), approximately 2/3 of whom had radiological changes, most commonly both pleural and parenchymal, 63% had disturbed lung function and 38#, clubbing. Those with radiological abnormality 93 (37%) had lower values for TLco and VC than those without. Either pleural or parerchymal abnormality occurred in Isolation in 43 (46%). Clubbing occurred in 35% of those with both changes together who often had other abnormalities and the lowest mean values for TLco and VC. Airways obstruction was found in some individuals with asbestosis. Pulmonary function and pleural or parenchymal abnormality are probably the earliest signs of asbestosis, rales, clubbing and pleural and parenchymal changes together, appear later. No evidence for more asbestosis in smokers than non-smokers was found. Doubt was cast on the methods of certification as some certified workers had minimal grounds for certification, other uncertified workers had good evidence of the disease. An objective method for evaluating the results of any individual was obtained by using principal component analysis. TLco and VC were the best indicators of general lung disease, FEV/VC%, cough and phlegm of airways obstructive disease and radiological abnormality, rales and finger clubbing of asbestosis. A correlation between exposure and disease (as measured by component l) in factories B and U but not in factory M suggested that chrysotile, used at M, is less dangerous than amosite (at U) and amosite plus croeidolite (at B). Lack of comparable past dust figures in the three factories casts doubt on this conclusion.