The cytochrome P450 3A (CYP3A) subfamily members are the most abundant and important drug-metabolizing enzymes in humans, and wide interindividual variability in CYP3A expression and function is present. CYP3A4 alone cannot fully explain the observed constitutive variability because its genetic variants are relatively uncommon and have limited functional significance, whereas CYP3A5 expression in humans is highly variable and may be contributory. However, it is difficult to delineate the relative contribution of CYP3A4 and CYP3A5, and to differentiate their effects on drug metabolism as their protein structure, function and substrates are so similar. By contrast, molecular biology methods provide the ability to identify CYP3A4 and CYP3A5 genotypes with certainty. This review collates currently available data on CYP3A5 polymorphisms, provides information on the population frequency of each genetic variant in major ethnic groups, and describes in vitro and in vivo studies that have attempted to identify genotype-phenotype associations.