Ethnic differences in drug metabolism are well documented for a number of drugs. The molecular mechanisms responsible for ethnic differences in drug metabolism have been partly clarified because of the advances in molecular biology in recent years. Gene dosage determines the drug metabolism as demonstrated for S-mephenytoin and diazepam metabolism. Genotype analysis indicates a different frequency for the mutant alleles in different ethnic populations, which results in variations in the frequency of subjects who are homozygous for the mutant allele among the extensive metabolizers in different ethnic populations. Ethnic differences in drug metabolism may result from differences in distribution of a polymorphic trait and mutations which code for enzymes with abnormal activity which occur with altered frequency in different ethnic groups.