Previous in vivo and in vitro studies have suggested that vitamin D₃ may have chemopreventive and chemotherapeutic properties in the management and treatment of androgen-dependent and androgen-independent prostate cancer. To investigate the therapeutic potential of vitamin D3 for prostate cancer, we have utilized the LPB-Tag transgenic model of prostate cancer and a vitamin D receptor-null (VDRKO) mouse model to determine the effects of high dietary calcium on tumor progression, and the role of the vitamin D receptor (VDR) in prostate tumor progression. Altered levels of dietary calcium did not change tumor progression in the LPB-Tag mouse model, refuting many epidemiological studies, and providing the groundwork for the use of a high calcium rescue diet in later studies of VDRKO mice. In animals with low levels of circulating testosterone and in the absence of exogenouse testosterone supplementation, the VDR confers protection against tumor progression. In the presence of exogenous testosterone, tumor morphology is not different in VDR wild-type (VDRWT) or VDR knock-out (VDRKO) tumors, suggesting a loss of the protective effects against tumor progression. VDRKO tumors show increased epithelial proliferation, resulting in a larger epithelial compartment. Conversely, VDRWT tumors have a higher level of epithelial apoptosis, and thus have a more pronounced stromal compartment. Laser capture microdissection of the epithelial and stromal layers of VDRWT and VDRKO tumors and subsequent quantitative RT-PCR shows an overall increase in genes promoting cell cycling, such as cyclin-dependent kinase 2 (cdk2) and survivin in VDRKO epithelium and stroma over both VDRWT and non-transgenic tissues. Epithelial-stromal signaling also appears to be enhanced in VDRKO epithelium through upregulation of the growth factor receptors IGFR-1 and IGFR-2. Loss of the VDR, and thus vitamin D₃ action correlates with increased cell proliferation, decreased cell death, and a potential increase in paracrine growth factor signaling; this correlation appears to be affected by androgen signaling, and has important implications in co-treatment administration of anti-androgens and vitamin D₃ for the treatment of prostate cancer.