PPM1D is an oncogene that is amplified and overexpressed in many human tumors, including breast cancer. It functions as a negative regulator of the p38 MAP kinase-p53 signaling pathway and is also proposed to participate in other critical cell survival pathways. To define the functional significance of PPM1D specifically in breast cancer, we used RNA interference to inhibit PPM1D expression in BT-474, MCF7, and ZR-75-1 breast cancer cell lines harboring amplification and increased expression of PPM1D. Efficient downregulation of PPM1D resulted in significantly reduced cell proliferation in MCF7 and ZR-75-1 cells carrying wild-type p53 but not in BT-474 carrying mutant p53, which indicates that the antiproliferative effect of PPM1D silencing is dependent on the p53 status of the cells. This result is in excellent agreement with the notion that PPM1D activation is an alternative mechanism for p53 inactivation. Additionally, our data indicate that the reduced cell growth observed after PPM1D silencing is due at least in part to increased apoptotic cell death. Our findings demonstrate that PPM1D is involved in the regulation of cell proliferation in breast cancer in a p53-dependent manner and that overexpression of PPM1D contributes to malignant phenotype by promoting sustained cell growth and cell survival.