Chromosome 10 deletions are among the most common genetic changes in highly malignant glial tumors. It has been noted that loss of heterozygosity (LOH) at 10q23 is a frequent alteration in a variety of human tumors and occurs in approximately 70% of all glioblastomas. By mapping of homozygous deletions on 10q23, a candidate tumor suppressor gene has been isolated, called PTEN for "phosphatase and tensin homolog deleted on chromosome 10" and MMAC1 for "mutated in multiple advanced cancers-1." Mutations of this tumor suppressor gene PTEN/MMAC1 have been reported in anaplastic glial tumors. The objective of this paper was to individuate a prognostic marker in exons 5, 6, 7, and 8 of the PTEN/MMAC1 gene for the high-grade malignant glioma with the most aggressive clinical behavior. In this study, we undertook sequence analysis of these exons in six selected patients with high-grade malignant gliomas who underwent radical aggressive tumor resection followed by radiotherapy within 3 weeks after surgery and subsequent chemotherapy. In them, the exon 5 sequence of the PTEN/MMAC1 gene is suggestive of a genetic survival marker in gliomas with high-grade malignancy.