There is increasing information available on the existence of polymorphisms in genes encoding xenobiotic metabolizing enzymes and the functional significance of many of these. In addition to genes long recognized as being polymorphic, such as CYP2D6, CYP2C19 and CYP2C9, there is now information available on the existence of polymorphisms in other cytochrome P450 genes such as CYP2A6, CYP2B6 and CYP2C8. With respect to phase II metabolism, polymorphisms in GSTM1, GSTT1, NAT2 and TPMT are well understood but information is also emerging on other GST polymorphisms and on polymorphisms in the UDP-glucuronosyltransferases and sulfotransferases. The availability of comprehensive information on the occurrence and functional significance of polymorphisms affecting drug metabolism should facilitate their application to pharmacogenomic profiling.