In addition to its important role in the maintenance of the skeleton, there is mounting evidence that vitamin D has effects on other body systems, and that adequate supplies of vitamin D are likely to be required for optimal health. Vitamin D is obtained both from dietary sources and from cutaneous synthesis with exposure to sunlight. Some epidemiological studies have indicated that vitamin D deficiency and decreased exposure to solar UVB radiation increase the risk of some cancers, including breast cancer. The active metabolite of vitamin D, 1,25-dihydroxy-vitamin D(3), is synthesized primarily in the kidney, and has been shown in laboratory studies to have potent anti-proliferative effects on breast cancer cells. Normal and neoplastic breast tissues contain the vitamin D receptor, and gene ablation studies have implicated the receptor in normal breast development. Several polymorphisms have been identified in the vitamin D receptor gene, and these have been associated with risk of breast cancer in some studies. Local synthesis of 1,25-dihydroxyvitamin D(3) in breast tissue may contribute to maintenance of normal cell function, which could be impaired in vitamin D deficiency.