Giant cell glioblastoma is a rare glioblastoma variant characterized by the presence of large, bizarre, multinucleated giant cells. This glioblastoma subtype develops clinically de novo after a short clinical history and contains a high frequency of p53 mutations. In this study, we screened a series of 18 giant cell glioblastomas for additional genetic alterations. PCR-SSCP followed by DNA sequencing revealed PTEN mutations in 5 of 15 tumors (33%). Of these, two mutations were located in exon 5, two mutations in exon 6, and one mutation each in exons 1 and 9. Four mutations were point mutations and two mutations were deletions. One neoplasm contained two PTEN mutations (exons 5 and 6). None of the giant cell glioblastomas showed a homozygous deletion of PTEN orp16, or amplification of MDM2. Immunohistochemically, MDM2 overexpression was either not observed or detected in only a minor fraction of tumor cells. Differential PCR revealed EGFR amplification in only one of 17 tumors (6%). These results indicate that giant cell glioblastomas occupy a hybrid position, sharing with primary (de novo) glioblastomas a short clinical history, the absence of a less malignant precursor lesion and a 30% frequency of PTEN mutations. With secondary glioblastomas that develop through progression from low-grade astrocytomas, they have in common a younger patient age at manifestation and a high frequency (>70%) of p53 mutations.