In vitro, calcitriol, the most potent metabolite of vitamin D, inhibits a variety of cellular pathways that promote cell proliferation and survival. Vitamin D has been shown to reduce the growth of breast cancer precursor cells in cell culture studies. In animal models Vitamin D has been shown to prevent the growth and progression of transplanted cell lines MCF710A, which is a model of pre-invasive cancer. Serum Vitamin D level deficiency correlates with an increased risk of breast cancer, and reduced survival of breast cancer patients. Vitamin D is also recognized to have effects on immune cell function and autoimmunity. The safety profile of oral Vitamin D, and its metabolite calcitriol, was well established for moderate term and acute therapy worldwide. Potential additional primary and secondary benefits of vitamin D are a) the suppression of carcinogen-induced transformation or progression of breast epithelium, and b) the enhancement of innate immune defense of pre-invasive breast cancer lesions, and c) its qualification as a combination therapy when combined with other neoadjuvant therapies for DCIS.