1α,25-Dihydroxyvitamin D 3 (1α,25(OH) 2 D 3 ) and its derivatives are a potential treatment of human prostate cancer. The antiproliferative action of 1α,25(OH) 2 D 3 is mainly exerted through nuclear vitamin D receptor (VDR)-mediated control of target gene transcription. To explore the target genes which are regulated by 1α,25(OH) 2 D 3 in human prostate cancer LNCaP cells, cDNA microarray was performed by using a chip that contains 3000 gene probes. The results showed that 24 genes were regulated by 1α,25(OH) 2 D 3 . Five of them encode proteins which belong to metabolic enzymes and fatty acid biosynthesis. Fatty acid synthase (FAS) was found to be down-regulated by 1α,25(OH) 2 D 3 , and the regulation was confirmed by real-time quantitative RT-PCR analysis. Inhibition of FAS expression by 1α,25(OH) 2 D 3 in LNCaP cells was more than 50% at 6h. Inhibitory effect of 1α,25(OH) 2 D 3 on FAS expression was completely blocked in the presence of protein synthesis inhibitor cycloheximide, indicating that the down-regulation of FAS gene expression by 1α,25(OH) 2 D 3 was indirect in LNCaP cells. An inhibition of FAS activity by cerulenin resulted in a strong inhibition of LNCaP cell proliferation. The inhibition of FAS expression and cell proliferation by 1α,25(OH) 2 D 3 seemed to be androgen-dependent, since antiandrogen, casodex and DCC-treatment of serum blocked the vitamin D action. The findings suggest that FAS is involved in the antiproliferative effect of 1α,25(OH) 2 D 3 in presence of androgens on prostate cancer LNCaP cells.