Three groups of men and women employed at a Rochdale asbestos textile factory have been followed to 30 June 1983 and their mortality compared with that expected at national and Rochdale rates, i.e. (i) 145 men first employed before 1933 who had served 20 yr or more in scheduled areas, (ii) 283 women first employed in 1933 or later who had served ten or more years in scheduled areas, and (iii) a principal cohort of 3211 men first employed between 1933 and 1974. Systematic measurements of the ambient pollution in the workplace have been made since 1951 either in terms of particles ml −1 or of fibres (of specified size) ml −1 and estimates have been made of the average concentration of particles (or fibres) to which individual members of the principal cohort were exposed. From these data estimates have been made of the quantitative relationship between ambient exposure and mortality from lung cancer and mesothelioma. These results, together with other published data, suggest that the lung cancer rate among chrysotile textile workers is approximately doubled following a cumulative exposure of 100 fibre ml −1 yr, irrespective of age at first exposure. Chrysotile asbestos was used in the factory throughout, but some crocidolite (approx. 5.0%) was used between 1932 and 1968 and this may have contributed, perhaps substantially, to the observed mesothelioma risk. For the purpose of risk prediction we have rather arbitrarily assumed that the mesothelioma risk caused by a similar concentration of pure chrysotile would have been roughly half that observed in this factory. This implies that 35 yr chrysotile exposure at an average concentration of 1 fibre ml −1 might eventually cause mesothelioma in about one worker in 200 if exposure began at age 20, although the risk would be substantially lower if exposure began later in life. If enforcement of the current U.K. control limit for chrysotile of 0.5 fibre ml −1 results in an average level of 0.25 fibre ml −1 in the workplace, these estimates suggest that the risk of eventually developing asbestos-related lung cancer or mesothelioma to a man first employed at age 20 who works in the asbestos textile industry for 35 yr may be about 0.8%. This prediction is of doubtful accuracy, however, as the exposure data were derived largely from static particle counts and the correlation between particle and fibre counts is poor.