Clear and conclusive evidence exists to implicate arsenic, chromium, and nickel in human carcinogenesis. Some evidence also exists to implicate cadmium as having caused human cancer. A number of epidemiological studies conducted with workers exposed to carcinogenic metals, as well as with those members of the general population who have had high exposure chiefly to these metals and their compounds, suggest that they possess human carcinogenic activity. The number and reliability of scientific conclusions that can be drawn from such epidemiological studies are limited because neither dosage nor route of exposure can usually be reliably assessed. These studies do, however, involve human cancers, a fact that amplifies the special importance of their results since human experiments cannot be performed for ethical reasons.