Aromatic and heterocyclic amines require metabolic activation to electrophilic intermediates that initiate carcinogenesis. N-Acetyltransferase 1 (NAT1) and 2 (NAT2) are important enzymes in the biotransformation of these carcinogens and exhibit genetic polymorphism. Human NAT1 and NAT2 alleles are listed at: http://www.louisville.edu/medschool/pharmacology/NAT.html by an international gene nomenclature committee. The high frequency of the NAT1 and NAT2 acetylation polymorphisms in human populations together with ubiquitous exposure to aromatic and heterocyclic amines suggest that NAT1 and NAT2 acetylator genotypes are important modifiers of human cancer susceptibility. For cancers in which N-acetylation is a detoxification step such as aromatic amine-related urinary bladder cancer, NAT2 slow acetylator phenotype is at higher risk. Multiple studies have shown that the urinary bladder cancer risk is particularly high in the slowest NAT2 acetylator phenotype or genotype (NAT2(*)5). In contrast, for cancers in which N-acetylation is negligible and O-acetylation is an activation step such as for heterocyclic amine-related colon cancer, NAT2 rapid acetylator phenotype is at higher risk. Although studies have found associations between NAT1 genotype and various cancers, the findings are less consistent and are not well understood. Since cancer risk requires exposure to aromatic and/or heterocyclic amine carcinogens modified by NAT1 and/or NAT2 acetylator genotype, the results from human epidemiology studies are dependent upon the quality and accuracy of the exposure assessment and genotype determination. Conclusions require understanding the relationship between genotype and phenotype, as well as the role of genetic variation in carcinogen metabolism, DNA repair, and host susceptibility. Investigations have been carried out in rapid and slow acetylator rodent models in which both exposure and genetic variability are tightly controlled. Human NAT1 and NAT2 alleles have been characterized by recombinant expression to further understand the effects of nucleotide polymorphisms on function and phenotype.