The major determinants of cancer risk are environmental carcinogens and predisposing host factors. Interactions between these determinants are now being studied by clinical-laboratory investigations. An individual may be predisposed to cancer by both genetic and acquired conditions. Genes related to malignancy and to carcinogenesis have been mapped to individual chromosomes in several species, including humans. Because most environmental chemical carcinogens require enzymatic activitation and a wide variation in carcinogen metabolism among people has been found, the ratio of metabolic activation to deactivation of carcinogens may ascertain, in part, a person's cancer risk. Additionally, cancer risk of chemical and physical agents can be qualitatively predicted by carcinogenicity tests in experimental animals. The emerging field of cancer ecogenetics is aiding efforts to prevent human cancer.