Loss of p53 function is involved in tumorigenesis of various human cancers, but the relation between mutation of the p53 tumor-suppressor gene and the chemo- and radiosensitivity of tumors remains unclear. Mutated p53 gene in malignant glioma is often associated with progression and recurrence of malignancy, and these events are closely linked with increased resistance to both chemotherapy and radiation. We have examined the status of the p53 gene in malignant gliomas obtained from 34 patients (glioblastoma: 29 cases, anaplastic astrocytomas: 5 cases). The chemosensitivities of these specimens using 28 kinds of anti-cancer agents were determined using an in vitro assay system. Overall, 12 mutated cases of p53 gene were found in malignant glioma samples. The mean numbers of effective agents were 0.58 for the tumor samples with p53 mutations and 5.00 for tumors without mutations. Our data indicate that p53 gene mutation predisposes to decreased cell killing via chemotherapy in malignant gliomas.