Cytochrome P450 (P450) enzymes are abundantly expressed in the human liver where they hydroxylate organic substrates. In a microarray screen performed in human liver cells, we found a group of eleven P450 genes whose expression was induced by p53 (CYP3A4, CYP3A43, CYP3A5, CYP3A7, CYP4F2, CYP4F3, CYP4F11, CYP4F12, CYP19A1, CYP21A2 and CYP24A1). The mode of regulation of four representative genes (CYP3A4, CYP3A7, CYP4F2 and CYP4F3) was further characterized. The genes were induced in a p53-dependent manner in HepG2 and Huh6 cells (both are cancer-derived human liver cells) and in primary liver cells isolated from human donors. Furthermore, p53 was found to bind to p53-responsive elements in the genes' DNA-regulatory regions and to enhance their transcription in a reporter gene assay. Importantly, when p53 was activated following the administration of either of three different anticancer chemotherapeutic agents (cisplatin, etoposide or doxorubicin), it was able to induce CYP3A genes, which are the main factors in systemic clearance of these agents. Finally, the p53-dependent induction of P450 genes following either Nutlin or chemotherapy treatment led to enhanced P450 enzymatic activity. Thus, in addition to the well-established role of p53 at the tumor site, our data unravels a novel function of hepatic p53 in inducing P450 enzymes and position p53 as a major factor in the hepatic response to xenobiotic and metabolic signals. Importantly, this study reveals a novel pathway for the induction of CYP3As by their substrates through p53, warranting the need for careful consideration when designing systemically administered chemotherapeutic regimens.