The p53 tumor suppressor protein is frequently mutated in human tumors. It is thought that the p53 pathway is indirectly impaired in the remaining tumors, for example by overexpression of its important regulators Mdm2 and Mdm4, making them attractive targets for the development of anti-cancer agents. Recent studies have suggested that Mdm4 levels determine the sensitivity of tumor cells for anti-cancer therapy. To investigate this possibility, we studied the drug sensitivity of several breast cancer cell lines containing wild-type p53, but expressing different Mdm4 levels. We show that endogenous Mdm4 levels can affect the sensitivity of breast cancer cells to anti-cancer agents, but in a cell line-dependent manner and depending on an intact apoptotic response. Furthermore, treatment with the non-genotoxic agent Nutlin-3 sensitizes cells for doxorubicin, showing that activation of p53 by targeting its regulators is an efficient strategy to decrease cell viability of breast cancer cells. These results confirm a function of Mdm4 in determining the efficacy of chemotherapeutic agents to induce apoptosis of cancer cells in a p53-dependent manner, although additional undetermined factors also influence the drug response. Targeting Mdm4 to sensitize tumor cells for chemotherapeutic drugs might be a strategy to effectively treat tumors harboring wild-type p53.