Mdm2 is often overexpressed in tumors that retain wild-type TP53 but may affect therapeutic response independently of p53. Herein is shown that tumor cells with MDM2 amplification are selectively resistant to treatment with topoisomerase II poisons but not other DNA damaging agents. Tumor cells that overexpress Mdm2 have reduced DNA double-strand breaks in response to doxorubicin or etoposide. This latter result is not due to altered drug uptake. The selective attenuation of DNA damage in response to these agents is dependent on both Mdm2 levels and an intact ubiquitin ligase function. These findings reveal a novel, p53-independent activity of Mdm2 and have important implications for the choice of chemotherapeutic agents in the treatment of Mdm2-overexpressing tumors.