Substantial interindividual differences occur for the metabolism of drugs, carcinogens, and steroid hormones, and these person-to-person differences are caused by genetic and environmental factors. It is likely that interindividual differences in the metabolism of xenobiotics and steroid hormones play a role in explaining interindividual differences in the initiation and progression of some human cancers. Factors that influence the metabolism and action of xenobiotics in human beings include age, disease states, hormonal changes in the body, ingestion of medicinal agents, exposure to environmental chemicals, and changes in life style, including factors such as cigarette smoking, alcohol consumption, and diet. Some individuals have a much larger response to environmental perturbations than do other individuals, and further research is needed to elucidate the reasons for differences in the responsiveness of people to modulators of chemical biotransformations. Immigration to a new country can represent a substantial change in a person's diet, life style, and environment, and these changes may alter the metabolism of xenobiotics and endogenous hormones that play a role in the carcinogenic response.