Among the four cytochrome P450 (CYP)3A genes, CYP3A4 and CYP3A5 are the most abundantly expressed in the human liver. Eighty single nucleotide polymorphisms (SNPs) of CYP3A4/5 have been reported to the Human P450 Allele Nomenclature Committee. CYP3A4 alleles with minimal function compared with wild type include the CYP3A4*6 and CYP3A4*17. Alleles with moderately decreased or altered activity include: CYP3A4*2, *8, *11, *12, *13, *16, and *18. CYP3A5 alleles with minimal function include the splice variants CYP3A5*3, *5, *6 and CYP3A5* 10, as well as the null allele CYP3A5*7. Alleles with moderately decreased catalytic activity include CYP3A5*8 and CYP3A5*9. This report reviews the current progress in the functional characterization of CYP3A4 and CYP3A5 SNPs and provides genotyping tests for possible defective variants. A combination of genotyping tests for defective CYP3A4/CYP3A5 haplotypes will be necessary to understand the variations in the metabolism and clinical toxicity of a wide variety of clinical drugs, since these two CYP proteins have overlapping substrate specificities.