Overcoming the resistance of glioblastoma cells against temozolomide, the first-line chemotherapeutic agent of choice for newly diagnosed glioblastoma, is a major therapeutic challenge in the management of this deadly brain tumor. The gene encoding O6-methylguanine DNA methyltransferase (MGMT), which removes the methyl group attached by temozolomide, is often silenced by promoter methylation in glioblastoma but is nevertheless expressed in a significant fraction of cases and is therefore regarded as one of the most clinically relevant mechanisms of resistance against temozolomide. However, to date, signaling pathways regulating MGMT in MGMT-expressing glioblastoma cells have been poorly delineated. Here in this study, we provide lines of evidence that the mitogen-activated protein/extracellular signal-regulated kinase kinase (MEK)–extracellular signal-regulated kinase (ERK)--murine double minute 2 (MDM2)-p53 pathway plays a critical role in the regulation of MGMT expression, using stem-like glioblastoma cells directly derived from patient tumor samples and maintained in the absence of serum, which not only possess stem-like properties but are also known to phenocopy the characteristics of the original tumors from which they are derived. We show that, in stem-like glioblastoma cells, MEK inhibition reduced MDM2 expression and that inhibition of either MEK or MDM2 resulted in p53 activation accompanied by p53-dependent downregulation of MGMT expression. MEK inhibition rendered otherwise resistant stem-like glioblastoma cells sensitive to temozolomide, and combination of MEK inhibitor and temozolomide treatments effectively deprived stem-like glioblastoma cells of their tumorigenic potential. Our findings suggest that targeting of the MEK-ERK-MDM2-p53 pathway in combination with temozolomide could be a novel and promising therapeutic strategy in the treatment of glioblastoma. STEM CELLS 2011;29:1942–1951.