Prostate cancer (PCa) cells harbor receptors for vitamin D (VDR) as well as androgens (AR). 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] increases AR expression and enhances androgen actions linking the two receptor systems. 1,25(OH)2D3 exhibits antiproliferative activity in both AR-positive and AR-negative PCa cells. Less calcemic analogs of 1,25(OH)2D3, with more antiproliferative activity, are being developed and will be more useful clinically. The mechanisms underlying differential analog activity are being investigated. In target cells, 1,25(OH)2D3 induces 24-hydroxylase, the enzyme that catalyzes its self-inactivation. Co-treatment with 24-hydroxylase inhibitors enhances the antiproliferative activity of calcitriol. Primary cultures of normal or cancer-derived prostatic epithelial cells express 1alpha-hydroxylase, the enzyme that catalyzes the synthesis of 1,25(OH)2D3, the levels being much lower in the cancer-derived cells and in PCa cell lines. This finding raises the possibility of using 25-hydroxyvitamin D3 [25(OH)D3] as a chemopreventive agent in PCa. In LNCaP human PCa cells, 1,25(OH)2D3 and its analogs exert antiproliferative activity predominantly by cell cycle arrest, but also induce apoptosis, although to a much lesser degree. Growth arrest is mediated by induction of IGF binding protein-3 (IGFBP-3), which in turn increases the expression of the cell cycle inhibitor p21, leading to growth arrest. Other actions of 1,25(OH)2D3 in PCa cells include promotion of pro-differentiation effects and inhibition of tumor cell invasion, metastasis and angiogenesis. Combination therapy with retinoids, other anticancer agents or 24-hydroxylase inhibitors augments the inhibitory activity of 1,25(OH)2D3 in PCa and provides another effective approach in PCa treatment. Small clinical trials have shown that 1,25(OH)2D3 can slow the rate of prostate specific antigen (PSA) rise in PCa patients, demonstrating proof of concept that 1,25(OH)2D3 or its analogs will be clinically effective in PCa therapy. Current research involves further investigation of the role of 1,25(OH)2D3 and its analogs for the therapy or chemoprevention of PCa.