In this GWAS of vitamin D serum levels, we identified two regions strongly associated with serum 25(OH)D—one on chromosome 4 surrounding the GC gene and the second on chromosome 11 including SNPs from CYP2R1, COPB1, PSMA1, and PDE3B. The identified loci replicated in an independent sample of women selected because they later developed breast cancer and the SNPs were also strongly associated with 25(OH)D concentrations measured in second blood samples from the same participants collected 3–10 years after baseline. Here, PSMA1 is linked to breast cancer.