Vitamin D and its metabolites are best known for their actions in calcium and bone metabolism. However, epidemiological studies have suggested that an increased prostate cancer risk is associated with decreased production of vitamin D. In vitro and in vivo studies have shown that the biologically active form of vitamin D, 1alpha,25-dihydroxyvitamin D3 (1,25D), inhibits proliferation of cancer cells derived from multiple tissues, including the prostate. Although the mechanisms underlying the growth inhibitory effects of 1,25D have not been fully elucidated, in prostate cancer cells 1,25D reduces cell growth via a number of cellular pathways, including cell cycle arrest, induction of apoptosis, and altered activation of growth factor signaling. The hypercalcemia induced by 1,25D in vivo limits its use clinically as a therapeutic agent. However, several 1,25D analogs have been developed that reduce prostate tumor growth in rodent xenograft models without causing hypercalcemia. Additional studies are required in order to determine whether these 1,25D analogs will be useful therapeutic agents for the treatment of prostate cancer.