Genomic alterations leading to aberrant activation of cyclin/cyclin-dependent kinase (cdk) complexes drive the pathogenesis of many common human tumor types. In the case of glioblastoma multiforme (GBM), these alterations are most commonly due to homozygous deletion of p16^INK4a and less commonly due to genomic amplifications of individual genes encoding cyclins or cdks. Here, we describe deletion of the p18^INK4c cdk inhibitor as a novel genetic alteration driving the pathogenesis of GBM. Deletions of p18^INK4c often occurred in tumors also harboring homozygous deletions of p16^INK4a. Expression of p18^INK4c was completely absent in 43% of GBM primary tumors studied by immunohistochemistry. Lentiviral reconstitution of p18^INK4c expression at physiologic levels in p18^INK4c-deficient but not p18^INK4c-proficient GBM cells led to senescence-like G₁ cell cycle arrest. These studies identify p18^INK4c as a GBM tumor suppressor gene, revealing an additional mechanism leading to aberrant activation of cyclin/cdk complexes in this terrible malignancy. [Cancer Res 2008;68(8):2564–9]