Vitamin D is emerging as an important dietary factor that affects the incidence and progression of many malignancies including prostate cancer. The active form of vitamin D, 1,25-dihydroxycholecalciferol [1,25(OH)(2)D(3)], inhibits the growth and stimulates the differentiation of prostate cancer cells. We have studied primary cultures of normal and cancer-derived prostatic epithelial cells as well as established human prostate cancer cell lines to elucidate the molecular pathways of 1,25(OH)(2)D(3) actions. These pathways are varied and appear to be cell specific. In LNCaP cells, 1,25(OH)(2)D(3) mainly causes growth arrest through the induction of insulin-like growth factor binding protein-3 and also stimulates apoptosis to a much smaller extent. We have used cDNA-microarray analyses to identify additional genes that are regulated by 1,25(OH)(2)D(3) and to raise novel therapeutic targets for use in the chemoprevention or treatment of prostate cancer. Less calcemic analogs of 1,25(OH)(2)D(3) that have more antiproliferative activity are being developed that will be more useful clinically. In target cells, 1,25(OH)(2)D(3) induces 24-hydroxylase, the enzyme that catalyzes its self inactivation. Cotreatment with 24-hydroxylase inhibitors enhances the antiproliferative activity of 1,25(OH)(2)D(3). The combination of other anticancer agents such as retinoids with vitamin D offers another promising therapeutic approach. A small clinical trial has shown that 1,25(OH)(2)D(3) can slow the rate of prostate-specific antigen increase in prostate cancer patients, which demonstrates proof of the concept that vitamin D or its analogs are clinically effective. Our research is directed at understanding the mechanisms of vitamin D action in prostate cells with the goal of developing chemoprevention and treatment strategies to improve prostate cancer therapy.