The first phase of the exploration of occupation-related cancer among machinists was a retrospective review of deaths among members of the International Association of Machinists and Aerospace Workers in 1973, in which mesotheliomas were identified in workers in railroad and other industries. The second phase of the study initiated in 1982 was the establishment of a cohort study of machinists, employed for railroad company A, who were alive in January 1954. The cohort consisted of 197 machinists who had been employed by the same railroad prior to 1935 and observed to 1982. Causes of death were identified for 132 of the cohort. There were 18 alive and 47 not traced. Among the 29 cancer deaths, there were nine mesotheliomas and one endothelioma of the pleura. Additional retrospective surveys of deaths among members of the railroad lodges of the international union, together with the cohort study, identified a total of 42 mesotheliomas, two endotheliomas of the pleura, and two cancers of the pleura among former railroad machinists. Among the machinists employed in other industries, 16 mesotheliomas and six cancers of the pleura were identified. For decades, machinists, by the nature of their craft, have had a high risk of occupation-related cancer due to asbestos exposure.