Estradiol-17 (E₂) induces rodent hippocamal neural progenitor cell (rNPC) proliferation in vitro (1, 2), in vivo (3, 4) and following brain injury (5). The purpose of the present investigation was to determine whether E₂-induced proliferation observed in rodent model systems generalized to cells of human neural origin and the signaling pathway by which E₂ promotes mitosis of human neural progenitor cells (hNPC). Results of these analyses indicate that E₂ induced a significant increase in hNPC proliferation in a time- and dose-dependent manner. E₂-induced hNPC DNA replication was paralleled by elevated cell cycle protein expression and centrosome amplification, which was associated with augmentation of total cell number. To determine whether estrogen receptor (ER) and which ER subtype were required for E₂-induced hNPC proliferation, ER expression was first determined by real time RT-PCR followed by Western blot analysis and subsequently verified pharmacologically using ER or -selective ligands. Results of these analyses indicated that ER expression was predominant relative to ER, which was barely detectable in hNPC. Activation of ER by the ER-selective ligand, diarylpropionitrile, led to an increase in phosphorylated extracellular signal-regulated kinase, and subsequent centrosome amplification and hNPC proliferation, which were blocked by the MEKK antagonist, UO126, but not its inactive analogue, UO124. These findings, for the first time, demonstrate the molecular cascade and related cell biology events involved in E₂-induced hNPC proliferation in vitro. Therapeutic implications of these findings relevant to hormone therapy and prevention of neurodegenerative disease are discussed.