The latency periods for formation of angiosarcoma of the liver due to exposures to vinyl-chloride-monomer (75014) (VCM) and thorium- dioxide (1314201) were compared. Data on thorium related liver angiosarcomas was obtained from reports issued in West Germany, the United Kingdom, Denmark, and the United States. Incidences of VCM related disease were determined from reports in 12 different countries. Exposures to thorium-dioxide occurred primarily between 1928 and 1955, while exposures to VCM were heaviest in the 1940 through 1960 period. The nature of the exposures was also different: thorium-dioxide exposures were continuous from the time of initial injection, while VCM exposures were intermittent. The median latency periods for angiosarcoma of the liver were 22 years for VCM and 29 years for thorium-dioxide. The authors conclude that the latency period for angiosarcoma of the liver is significantly shorter for VCM exposures compared with thorium-dioxide exposures. This difference appears to reflect differences in the mechanisms of chemical and radiation carcinogenesis, rather than differences in the populations. Cohort analyses of VCM and thorium-dioxide exposed populations are suggested to further clarify differences in the carcinogenic actions of the agents.