The estrogen receptor-α (ER-α) is a ligand-dependent transcription factor that regulates the growth, differentiation, and development of hormone-responsive target organs. While ER-α has been reported to play critical role in the pathogenesis and prognosis of breast and prostate cancers, its possible functional role in regulating prostate cancer cell growth in a ligand-dependent or -independent manner is poorly understood. We addressed this question by stably transfecting wild type (wt) ER-α cDNA into an invasive estrogen receptor-negative human prostate cancer cell line ARCaP. We isolated several clonal lines of transfected cells expressing varying levels of ER-α. The ectopic expression of wt ER-α markedly inhibited the growth of ARCaP cells in vitro in an ER-α dose-dependent but ligand-independent manner. Flow cytometric analysis of the wt ER-α-transfected ARCaP cells revealed that wt ER-α expression arrested cell growth in G1 phase. Our results suggest that ER-α may regulate prostate cell growth and participate in the pathogenesis of prostate cancer. ER-α may be delivered and expressed ectopically to target prostate cancer progression.