Between 1972 and 1996, the International Agency for Research on Cancer (IARC) identified 282 chemicals as having experimental evidence "sufficient" to conclude that the chemicals cause cancer in animals. Of the 282 IARC animal carcinogens, 7% (20) also had "sufficient" evidence from epidemiology studies. "Limited" human evidence was available for 5% (13) of the 282 IARC animal carcinogens. A 1996 IARC international compendium of cancer studies indicated that epidemiologic studies of chemicals determined by IARC to cause cancer in both humans and animals were considerably more common than studies of chemicals determined to cause cancer in animals but for which human data were inadequate or limited. Of the 1,101 studies reported to IARC by investigators, the IARC animal carcinogens, with the most ongoing studies were asbestos (50 studies) and benzene (26 studies). Both chemicals have sufficient evidence of carcinogenicity from studies of humans. There were epidemiologic studies ongoing for 65% (13/20) of the chemicals for which evidence of carcinogenicity was sufficient in both animals and humans. Epidemiologic studies were reported to IARC for 12% (30/247) of the IARC animal carcinogens for which human data were inadequate or for which no human data were available. Two factors appear critical in determining whether an IARC animal carcinogen becomes the subject of an epidemiologic study: the likelihood of overcoming methodological obstacles to carrying out successful epidemiologic studies, and the economic and/or regulatory interest in the chemical.