At similar smoking levels African American’s lung cancer risk is as much as twice that of whites. We hypothesized that racial/ethnic differences in UDP-glucuronosyltransferase (UGT)-catalyzed glucuronidation of 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL), a detoxication pathway for the tobacco-specific lung carcinogen NNK (4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone), may contribute to this variable risk. UGT2B10 catalyzes NNAL N-glucuronidation and a UGT2B10 splice variant is common among African Americans. Smokers from two independent studies were genotyped for this variant (rs116294140) and an Asp67Tyr variant (rs61750900), and urinary NNAL and NNAL glucuronide concentrations were quantified. In the first, no significant differences in NNAL-N-glucuronidation between African Americans (n=257) and whites (n=354), or between homozygous carriers of UGT2B10 variants (genetic score 2) and non-carriers (score 0) were detected. However, total NNAL glucuronidation by score 2 compared to score 0 smokers was lower (68.9% vs 71.2%, p<0.0001). To more precisely quantify NNAL N-glucuronide in a second study, a sensitive high resolution LC-MS/MS-based method, which separated NNAL, NNAL-O-glucuronide and NNAL-N-glucuronide prior to analysis, was developed. In this study, the excretion of total NNAL (free plus glucuronides) by African American (n=52) and white (n=54) smokers was not different, however, total NNAL glucuronidation by African Americans (64.0%) was slightly less than by whites (68.3%, p=0.05). The mean NNAL N-glucuronidation by African Americans was much lower than for whites (14% vs 24.9%, p<0.00001) but the NNAL O-glucuronidation was greater (50.0% vs 43.3%, p=0.013). UGT2B10 genotype influenced NNAL N-glucuronidation; the geometric mean percentage N-glucuronidation was 22.5% for smokers with genetic score 0 (n=57) and 11.2% for score 2 (n=11). In summary, the high prevalence of a UGT2B10 splice variant among African Americans results in lower NNAL N-glucuronidation but only a small decrease in total NNAL glucuronidation. Therefore despite the significant contribution of UGT2B10 to NNAL-N-glucuronidation, UGT2B10 genotype does not play a large role in NNAL detoxication. Any decrease in N-glucuronidation was accompanied by a parallel increase in O-glucuronidation.