Cytochrome P4501A2 (CYP1A2) has been identified as a key factor in the metabolic activation of numerous chemical carcinogens, including aflatoxin B1, various heterocyclic and aromatic amines, and certain nitroaromatic compounds. In addition, CYP1A2 contributes to the inactivation of several common drugs and dietary constituents, including acetaminophen and caffeine. Two xenobiotic-responsive-element (XRE)-like sequences and an antioxidant response element (ARE) have been identified in the regulatory region of the CYP1A2 gene; however, the functionality of the ARE remains to be demonstrated. Based on in vivo phenotyping assays, substantial interindividual variability in CYP1A2 activity has been reported. Some population-based studies have reported either bi- or tri-modal distributions in CYP1A2 phenotype, suggesting a genetic basis for the large interindividual differences in CYP1A2 activity. However, despite the polymodal distributions reported for CYP1A2 activity, a distinct functional genetic polymorphism in the gene has not been identified. Potential mechanisms contributing to the large interindividual variability in CYP1A2 activity are discussed. A thorough understanding of the functions and regulation of the CYP1A2 gene may ultimately lead to new methods for preventing or intervening in the development of certain chemically-related human cancers.