1,25-dihydroxyvitamin D3 (1,25VD), the active form of vitamin D (VD), suppresses the growth of numerous human cancer cell lines by inhibiting cell cycle progression and inducing cell death. Genes that mediate each of these activities remain largely unidentified and there are no preclinical data for 1,25VD analogues in ovarian cancer (OCa). We hypothesize that 1,25VD and its analogues inhibit the development of OCa. In this study, we demonstrated, (a) 1,25VD causes cell cycle arrest at the G1/S and G2/M transition and induces apoptosis in OCa cells. (b) We also found that gadd45 is one of primary target genes for 1,25VD-mediated G2/M arrest. A direct repeat 3 (DR3) vitamin D response element (VDRE) is identified in the fourth exon of gadd45. This exonic VDRE forms a complex with the vitamin D receptor (VDR)/retinoid X receptor (RXR) heterodimer in vitro and mediates the induction of reporter activity by 1,25VD in vivo. VDR is recruited in a ligand-dependent manner to the exonic enhancer but not to the gadd45 promoter regions. In OCa cells expressing GADD45 anti-sense cDNA or GADD45-null mouse embryo fibroblasts, 1,25VD fails to induce G2/M arrest, suggesting that G2/M arrest induced by 1,25VD is mediated through GADD45. Further study showed that GADD45 mediates the effect of 1,25VD by decreasing cdc2 kinase activity. (c) hTERT, the catalytic subunit of telomerase, is identified as a primary target for 1,25VD. 1,25VD decreases telomerase activity and hTERT mRNA expression. The down-regulation of hTERT mRNA is due to decreased mRNA stability by 1,25VD, rather than decreased transcription of hTERT through VDRE. Clones stably transfected with hTERT showed higher telomerase activity and longer telomere length than parental cells. Moreover, hTERT clones resist 1,25VD-induced apoptosis and growth inhibition. In contrast to parental cells which do not recover from prolonged treatment with 1,25VD, hTERT clones re-grew rapidly after 1,25VD withdrawal. (d) We demonstrated that the 1,25VD analogue EB1089 inhibits OCa cells in vitro and OCa xenograft in vivo without inducing hypercalcemia. We also demonstrated precursors for epithelial OCa express VDR and human primary ovarian surface epithelial cells respond to 1,25VD. Taken together, these results strongly suggest that 1,25VD analogues may be effective in the chemoprevention and chemotherapy of OCa.