The role of p53 gene mutations in the formation or progression of human astrocytic tumors is controversial. We studied the distribution pattern of p53 immunoreactivity and analyzed p53 gene mutations to define the significance of p53 gene mutations in astrocytoma tumorigenesis or malignant progression. Twenty-three astrocytic tumors were evaluated with immunohistochemistry, single-strand conformation polymorphism (SSCP) analysis, and sequence analysis. We also searched MEDLINE to collect data on p53 gene mutation frequencies in astrocytic tumors in order to evaluate the association of p53 mutations and tumor grade. Strong immunoreactivity with a diffuse clustering pattern was found in three of five glioblastomas and seven of 12 anaplastic astrocytomas. Three of four low-grade astrocytomas were immunonegative. The p53 immunopositive cells in the only positively staining low-grade astrocytoma in our study appeared sparsely scattered. The results of immunostaining suggested that clonal expansion was associated with astrocytoma progression. Mutations of the p53 gene were detected in four of the 23 astrocytomas (one glioblastoma and three anaplastic astrocytomas). In the genetic data analysis, 76 of 367 astrocytomas had p53 gene mutations. A significantly greater p53 gene mutation frequency was found in anaplastic astrocytomas or glioblastomas than in the low-grade astrocytomas. The results of these immunohistochemical and genetic studies support the view that p53 gene mutation is associated with the malignant progression from low-grade to high-grade astrocytomas rather than with tumor initiation or promotion.