A symposium, Genetic Polymorphisms in DNA Repair and Cancer Risk, was presented at the 40th Annual Meeting of the Society of Toxicology, held in San Francisco, California, in March 2001. A brief report of the symposium was published (Kaiser, Science 292, 837-838, 2001). Molecular epidemiological studies have shown that polymorphic variants of genes involved in the metabolism and repair of carcinogens can act as cancer susceptibility genes. These variants of drug metabolic and DNA-repair enzymes either increase the activation of chemical carcinogens or decrease the cells' ability to detoxify/repair mutagenic damages. Although on an individual basis these variant alleles may only slightly change catalytic activity and increase cancer risk, their polymorphic frequency in the human population may contribute to a high proportion of cancer cases. Studies conducted over the past few years have identified variant alleles for a number of DNA-repair genes, some of which have been shown to change DNA-repair capacity. Identifying these genotypic alterations in DNA-repair enzymes and their association with cancer may help to elucidate the mechanisms of cancer etiology and to predict both disease risk and response to cancer therapy, since most antineoplastic treatments mediate their effects through DNA damage.