Expression of oncogenic Ras in primary human cells activates p53, thereby protecting cells from transformation. We show that in Ras-expressing IMR-90 cells, p53 is phosphorylated at Ser33 and Ser46 by the p38 mitogen-activated protein kinase (MAPK). Activity of p38 MAPK is regulated by the p53-inducible phosphatase PPM1D, creating a potential feedback loop. Expression of oncogenic Ras suppresses PPM1D mRNA induction, leaving p53 phosphorylated at Ser33 and Ser46 and in an active state. Retrovirus-mediated overexpression of PPM1D reduced p53 phosphorylation at these sites, abrogated Ras-induced apoptosis and partially rescued cells from cell-cycle arrest. Inactivation of p38 MAPK (the product of Mapk14) in vivo by gene targeting or by PPM1D overexpression expedited tumor formation after injection of mouse embryo fibroblasts (MEFs) expressing E1A+Ras into nude mice. The gene encoding PPM1D (PPM1D, at 17q22/q23) is amplified in human breast-tumor cell lines and in approximately 11% of primary breast tumors, most of which harbor wildtype p53. These findings suggest that inactivation of the p38 MAPK through PPM1D overexpression resulting from PPM1D amplification contributes to the development of human cancers by suppressing p53 activation.