Background: Observational studies have suggested an association between circulating vitamin D concentrations [25(OH)D] and risk of breast and prostate cancer, which was not supported by a recent Mendelian randomization analysis comprising 15,748 breast and 22,898 prostate cancer cases. Demonstrating causality has proven challenging, and one common limitation of MR studies is insufficient power. Methods: We aim to determine if circulating concentrations of vitamin D are causally associated with the risk of breast and prostate cancer, by using summary level data from the largest-ever genome-wide association studies conducted on vitamin D (N=73,699), breast cancer (Ncase=122,977) and prostate cancer (Ncase=79,148). We constructed a stronger instrument using six common genetic variants (as compared with the previous four variants), and applied several two-sample MR methods.