This review briefly describes current understanding of one of the earliest discovered pharmacogenetic polymorphisms of drug biotransformation affecting acetylation of certain homo- and heterocyclic aromatic amines and hydrazines. This so-called acetylation polymorphism arises from allelic variation in one of the two known human arylamine N-acetyltransferase genes, namely NAT2, which results in production of NAT2 proteins with variable enzyme activity or stability. The NAT1 gene locus encodes a structurally related enzyme, NAT1, with catalytic specificity for arylamine acceptor substrates distinct from that exhibited by NAT2. NAT1 function is also genetically variable in human populations. Clinical and toxicological consequences of genetic variation in NAT1 and NAT2 activity are discussed.