OBJECTIVE The serum 25-hydroxyvitamin D (25(OH)D) level is not only dependent on vitamin D intake and production in the skin but also dependent on genetic factors. Thus, in large genome-wide association studies, it has been shown that single nucleotide polymorphisms (SNPs) in the vitamin D binding protein (DBP), as well as in enzymes related to activation or degradation of vitamin D and its metabolites, are as important for the serum 25(OH)D level as the effect of season. How these SNPs affect the serum 25(OH)D response to vitamin D supplementation is uncertain. METHODS Data were pooled from three randomized controlled trials where 40, 000 IU vitamin D/week was given for 6 months. Serum 25(OH)D was measured before and at the end of the intervention, and the subjects were genotyped for SNPs related to the serum 25(OH)D level. RESULTS Baseline 25(OH)D levels were significantly related to SNPs in the DBP and CYP2R1 genes. Those with SNPs associated with the lowest baseline 25(OH)D levels also had the smallest increase (delta) after supplementation. Those with the lowest baseline serum 25(OH)D (without regard to genotypes) had the highest increase (delta) after supplementation. Subjects with high BMI had lowest baseline 25(OH)D levels and also the smallest increase (delta) after supplementation. CONCLUSIONS The serum 25(OH)D response to supplementation depends on genes, baseline level, and BMI. However, whether this is clinically important or not depends on the therapeutic window of vitamin D, an issue that is still not settled.