Vitamin D metabolites are well-recognized to stop cancer cell growth in culture. However, a clear definition of a sufficient level of serum vitamin D (currently measured as 25(OH)D) for disease prevention has been hampered by inconsistent results from both observational studies and randomized clinical trials. Observational cancer studies report both increased and reduced risks of cancer in subjects with higher serum levels of 25(OH)D, while large randomized trials report no significant benefit of vitamin D supplementation for breast cancer (incidence), colon cancer (incidence and mortality), lung cancer (mortality), or benign proliferative breast disease.