Large interindividual variability in morphine disposition could contribute to unpredictable variability in morphine analgesia and adverse events. Caucasian children have more adverse effects and slower morphine clearance than African-American children. To study variations in intravenous morphine pharmacokinetics in children, we examined the influence of genetic polymorphisms in OCT1.In 146 children undergoing adenotonsillectomy, 146 concentration-time profiles (2-4 measurements per patient) were available. Population pharmacokinetic analysis characterized the profiles in NONMEM(®) and tested OCT1 variants as covariates.Allometrically scaled post hoc Bayesian morphine clearance in homozygotes of loss-of-function OCT1 variants (n = 9, OCT1*2-*5/*2-*5) was significantly lower (20%) than in wild-type (n = 85, OCT1*1/*1) and heterozygotes (n = 52, OCT1*1/*2-*5; p < 0.05).Besides bodyweight, OCT1 genotypes play a significant role in intravenous morphine pharmacokinetics. Relatively high allelic frequencies of defective OCT1 variants among Caucasians may explain their lower morphine clearance and possibly higher frequencies of adverse events compared with African-American children. Original submitted 21 December 2012; Revision submitted 7 May 2013.