Two lines of evidence indirectly implicate the tumor suppressor p53 (also known as TP53) gene in glioma development. First, germline mutations of the p53 gene are associated with increased susceptibility to glioma. Second, chromosome 17p deletions and p53 gene mutations are found frequently in suoradic gliomas of all malignancy stages. These observations suggest that mutations of the p53 gene may be early events in glioma development. Our purpose was to analyze 15 low-grade astrocytic gliomas that progressed to highergrade gliomas, examining the status of the p53 gene in both the initial and recurrent tumors. Also, we explored the relationships between p53 status, DNA ploidy, tumor grade, and patient survival. Fifteen low-grade gliomas that recurred as tumors of higher grade 17–102 months after initial treatment (biopsy, resection, radiotherapy, or chemotherapy) were identified from hospital records of patients (eight male and seven female) aged 31–68 years. Pathologic diagnosis was re-evaluated. Polymerase chain reaction (PCR)-single-strand conformation polymorphism and DNA sequencing were perfomed on tissue samples from the initial and recurrent tumors of each patient, using oligonucleotide PCR primers directed to exons 5–9 of the p53 gene. p53 expression was determined by immunohistochemistry and DNA ploidy evaluated by DNA flow cytometry. Eight (53%) of fifteen tumors had p53 mutations in exons 5–9. Nine (64%) of fourteenwere immunopositive initially, and eight of these were also immunopositive at recurrence. p53 gene status was significantly associated with p53 expression in the initial tumor ( P =.02), and p53 expression at initial diagnosis was significantly related to tumor pathology at recurrence ( P =.03). Patients with p53 mutant tumors survived nearly twice as long as those without mutations (median survival, 61 versus 33 months; P =.031). There was no significant difference in recurrence-free survival between patients with p53 mutant and nonmutant tumors (48 versus 33 months; P =.37), but there was a significant difference in postrecurrence survival (17 versus 2 months; P =.019). Low-grade tumors that recurred as anaplastic gliomas were characterized by p53 gene mutation, immunopositivity, and DNA nondiploidy. Low-grade tumors that recurred as glioblastomas generally had intact p53 genes and were immunonegative. These findings suggest that histologically indistinguishable, low-grade astrocytic gliomas that are destined to progress to higher grades, do so along two distinct clinicopathologic pathways (either stepwise to anaplastic glioma, then glioblastoma, or directly to glioblastoma) marked by the presence or absence of p53 mutation. [J Natl Cancer Inst 86: 1011–1017, 1994]