Active vitamin D increases the differentiation and exerts antiproliferative effects in cancer cells. Recent data suggest that vitamin D is activated locally in cancer cells. Ecologic studies have shown an inverse correlation between breast cancer mortality and sun exposure and dietary vitamin D intake. In clinical studies an impaired vitamin D status is associated with a 20-30% increased breast cancer incidence and 10-20% increased mortality. As vitamin D insufficiency is common, it is important to clarify whether vitamin D status affects the risk and prognosis of breast cancer.