Interindividual variability in drug response is a well-recognized problem resulting in both undertreatment and overtreatment of individuals receiving similar doses of drugs, with the potential for lack of therapeutic effect and for drug toxicity. The potential that interethnic differences might contribute to such interindividual variability in drug response has recently been recognized. Such interethnic differences in drug response may be due to either altered drug disposition or altered drug sensitivity among races at similar drug concentrations. In turn, such racial differences in drug disposition may be related to genetic or environmental factors, which are often difficult to separate.