There is wide inter-individual variability in the pharmacokinetics, pharmacodynamics and tolerance of anticancer drugs. Recent evidence suggests that there is even greater variability between individuals of different ethnicity. Allelic variants of genes encoding drug metabolising enzymes are expressed with different incidences in different ethnic groups, particularly between Asian and Caucasians, and some of these variants result in altered enzyme function. There is also preliminary evidence to suggest that ethnic differences in the expression of allelic variants may produce altered pharmacokinetics of anticancer drugs, including paclitaxel and irinotecan. Emerging evidence indicates that toxicity from certain anticancer treatments is much greater in Asian patients than Caucasians in breast and lung cancers. Understanding the causes of ethnic differences in cytotoxic metabolism may promote improved understanding of inter-individual differences in the pharmacokinetics and tolerance of cytotoxic drugs leading to improved and more individualised prescribing.