Among tumours of the nervous system, mutations of the p53 tumour suppressor gene are largely restricted to neoplasms of astrocytic origin. These are the most common human brain tumours and span a wide range of biologic behavior, from the slowly growing low-grade astrocytoma (WHO Grade II) to anaplastic astrocytoma (WHO Grade III) and, ultimately, the glioblastoma multiforme (WHO Grade IV). In low grade astrocytomas, p53 mutations with or without loss of heterozygosity on chromosome 17p are the principle detectable change. Anaplastic astrocytomas contain p53 mutations in approximately one third of cases and further display loss of heterozygosity on chromosome 19q and homozygous loss of 9p21, tentatively identified as multiple tumour suppressor 1 (MTS-1). In addition to these genetic alterations, glioblastomas show loss of chromosome 10 and amplification of the EGF receptor gene at an incidence of > 60% and > 40%, respectively. The type and distribution of p53 mutations are not suggestive of specific environmental carcinogens operative in their etiology.