Cultured cortical neurons exposed to the Human Immunodeficiency Virus gp120 coat protein undergo apoptosis involving activation of both caspase-8 and caspase-9. Additionally, gp120-mediated neuronal apoptosis requires the pro-apoptotic transcription factor p53. As caspase-8-induced apoptosis does not typically require p53, we examined the possibility of a novel role for p53 in caspase-8 activation initiated by gp120. We observed that gp120 treatment of cultured cortical neurons induced caspase-8 activity and Bid cleavage independently of p53, but induction of caspase-3 enzymatic activity required p53 expression. These findings suggested the possibility that p53 down-regulates a caspase-3 inhibitor. We observed high-level expression of the caspase-3/9 inhibitor X-linked inhibitor of apoptosis protein (XIAP) in cultured cortical neurons. Adenoviral expression of p53 or induction of endogenous p53 by camptothecin treatment reduced XIAP protein in neurons. Infection with a p53 expressing adenovirus increased expression of the mRNA for Omi/HtrA2, a protease that cleaves and inactivates XIAP. These findings suggest that p53 regulates neuronal apoptosis, in part, by suppressing the anti-apoptotic protein XIAP via transcriptional activation of Omi/HtrA2.