ABSTRACT The key enzyme responsible for β‐carotene conversion into retinal is β‐carotene 15,15′‐monoxygenase (BCMO1). Since it has been reported that the conversion of β‐carotene into vitamin A is highly variable in up to 45% of healthy individuals, we hypothesized that genetic polymorphisms in the BCMO1 gene could contribute to the occurrence of the poor converter phenotype. Here we describe the screening of the total open reading frame of the BCMO1 coding region that led to the identification of two common nonsynonymous single nucleotide polymorphisms (R267S: rs12934922; A379V: rs7501331) with variant allele frequencies of 42 and 24%, respectively. In vitro biochemical characterization of the recombinant 267S + 379V double mutant revealed a reduced catalytic activity of BCMO1 by 57% ( P <0.001). Assessment of the responsiveness to a pharmacological dose of β‐carotene in female volunteers confirmed that carriers of both the 379V and 267S + 379V variant alleles had a reduced ability to convert β‐carotene, as indicated through reduced retinyl palmitate:β‐carotene ratios in the triglyceride‐rich lipoprotein fraction [−32% ( P =0.005) and −69% ( P =0.001), respectively] and increased fasting β‐carotene concentrations [+160% ( P =0.025) and +240% ( P =0.041), respectively]. Our data show that there is genetic variability in β‐carotene metabolism and may provide an explanation for the molecular basis of the poor converter phenotype within the population.—Leung, W. C., Hessel, S., Méplan, C., Flint, J., Oberhauser, V., Tourniaire, F., Hesketh, J. E., vonLintig, J., Lietz, G. Two common single nucleotide polymorphisms in the gene encoding β‐carotene 15,15′‐monoxygenase alter β‐carotene metabolism in female volunteers. FASEB J . 23, 1041–1053 (2009)