Olanzapine (OLZ) is an antipsychotic used in the treatment of schizophrenia, bipolar disorder, and treatment-resistant depression. Glucuronidation via the UDP-glucuronosyltransferase (UGT) family of enzymes is the major mode of OLZ metabolism and polymorphisms in these enzymes could contribute to inter-individual variability in OLZ metabolism and therapeutic response. In a screening of cell lines over-expressing individual UGTs, only UGTs 1A4 and 2B10 exhibited glucuronidation activity against OLZ. The UGT1A424Pro/48Val variant exhibited a 3.7-fold (p<0.0001) higher Vmax/KM for formation of the OLZ-10-N-glucuronide isomer 1 and a 4.3-fold (p<0.0001) higher Vmax/KM for formation of the OLZ-10-N-glucuronide isomer 2 than wild-type UGT1A424Pro/48Leu. The UGT2B1067Y variant exhibited no glucuronidation activity against OLZ. In a screening of 105 human liver microsomal (HLM) specimens, there was a 2.1- (p=0.04) and 1.6- (p=0.0017) fold increase in the rate of OLZ-10-N-glucuronide isomer 1 and OLZ-4’-N-glucuronide formation, and a 2.0-fold (p=0.02) increase in overall OLZ glucuronidation formation, in HLM with the UGT1A4 (*3/*3)/UGT2B10 (*1/*1) genotype compared to HLM with the UGT1A4 (*1/*1)/UGT2B10 (*1/*1) genotype. There was a 1.9-fold (p<0.003) decrease in formation of both isomers of the OLZ-10-N-glucuronide, a 2.7-fold (p<0.0001) decrease in OLZ-4’-N-glucuronide formation, and a 2.1-fold (p=0.0002) decrease in overall OLZ-glucuronide formation in HLM with at least one UGT2B10*2 allele. In regression analysis, both the UGT1A4*3 (p<0.02) and UGT2B10*2 (p<0.002) alleles were significant predictors of formation of all OLZ-glucuronide isomers. These data suggest that the UGT1A4*3 and UGT2B10*2 alleles contribute significantly to inter-individual variability in OLZ metabolism.