Epidemiologic evidence on the relation between exposure to metals and cancer is reviewed. Human exposure to metals is common, with wide use in industry and long-term environmental persistence. Historically, the heaviest metal exposures occurred in the workplace or in environmental settings in close proximity to industrial sources. Among the general population, exposure to a number of metals is widespread but generally at substantially lower levels than have been found in industry. The carcinogenicity of arsenic, chromium, and nickel has been established. Occupational and environmental arsenic exposure is linked to increased lung cancer risk in humans, although experimental studies remain inconclusive. Experimental studies clearly demonstrate the malignant potential of hexavalent(VI) chromium compounds, with solubility being an important determining factor. Epidemiologic studies of workers in chromium chemical production and use link exposure to lung and nasal cancer. Experimental and epidemiologic data show that sparingly-soluble nickel compounds and possibly also the soluble compounds are carcinogens linked to lung and nasal cancer in humans. Some experimental and epidemiologic studies suggest that lead may be a human carcinogen, but the evidence is inconclusive. Although epidemiologic data are less extensive for beryllium and cadmium, the findings in humans of excess cancer risk are supported by the clear demonstration of carcinogenicity in experimental studies. Other metals, including antimony and cobalt, may be human carcinogens, but the experimental and epidemiologic data are limited.