Background : We report the determinants of serum levels of vitamin D in a UK melanoma case–control study benefitting from detailed exposure and genotyping data. ; Methods : Sun exposure, supplemental vitamin D, and SNPs reported to be associated with serum levels were assessed as predictors of a single serum 25-hydroxyvitamin D3 measurement adjusted for season, age, sex, and body mass index. ; Results : Adjusted analyses showed that vitamin D levels were sub-optimal especially in the sun-sensitive individuals (−2.61 nmol/L, p = 0.03) and for inheritance of a genetic variant in the GC gene coding for the vitamin D-binding protein (−5.79 for heterozygotes versus wild type, p =