A longitudinal study was undertaken of all workers in two crocidolite (12001284) and chrysotile (12001295) filter paper manufacturing facilities. The first facility, A, was used beginning in 1943 for the production of filters for military masks, as well as materials for civil defense and the needs of the Atomic Energy Commission. The second facility, B, used only Canadian chrysotile beginning in 1961 in the manufacture of gaskets. At facility-A the crocidolite was delivered in burlap bags and used openly. At facility-B chrysotile was delivered encapsulated in pulp sheets. The two groups exposed 15 to 27 years were quite similar with respect to age, sex, geographic location, manner of yearly clinical examination and minimum latent period of 15 years. The group from facility-A was never involved in the dry carding process which had been discontinued at this site in 1956. Even so, there was a striking difference between exposure to the two minerals even during the short period. In the crocidolite group there were three deaths from asbestosis at 17, 17, and 18 years of exposure; one death due to mesothelioma at 21 years of exposure; and four lung cancer deaths occurring at 12, 23, 24, and 26 years of exposure. No asbestos related deaths occurred among the chrysotile workers, although they were older and exposed for longer periods. The authors conclude that those exposed to crocidolite have a much worse experience with respect to the incidence of asbestosis, lung cancer and mesothelioma.