The N-acetyltransferases (NAT; E.C.2.3.1.5) are involved in the metabolism of drugs and environmental toxins. They catalyse the acetyl transfer from acetyl coenzyme A to an aromatic amine, heterocyclic amine, or hydrazine compound. NAT homologues are present in numerous species from bacteria to human. Sequence variations in the human NAT1 and NAT2 result in the production of NAT proteins with variable enzyme activity or stability, leading to slow or rapid acetylation. Therefore, genetic polymorphisms in NAT1 and NAT2 influence drug metabolism and drug-related toxicity. Epidemiological studies suggest that the NAT1 and NAT2 acetylation polymorphisms modify the risk of developing cancers of the urinary bladder, colorectal, breast, head and neck, and lung.