The incidence of primary brain tumors has increased dramatically among elderly North Americans during the past two decades. Numerous chromosomal abnormalities have been associated with these tumors; various subsets of these abnormalities are specific to certain types of brain tumors. Astrocytic gliomas may exhibit losses of genetic information from chromosomes 9p, 10q, 11p, 13q, 17p, or 22. Mutations of the p53 gene are found mostly in the malignant astrocytic forms and have been linked to malignant tumor transformation and progression. Functional and structural abnormalities of the neurofibromatosis 1 (NF1) gene and overexpression of the epidermal growth factor receptor have been associated with expression of the malignant glioma phenotype. Other less clearly defined abnormalities in astrocytomas include mutations of the retinoblastoma (RB) gene and overexpression of platelet-derived growth factor; transforming growth factor-alpha and -beta; the c-erb B-1, c-myc, ras, c-fos, and ros oncogenes; and insulin-like growth factor I and II. In other glioma tumors, p53 mutations are either infrequent, as in oligodendrogliomas, or absent, as in ependymomas. Occasionally, medulloblastomas exhibit p53 mutations and loss of genetic information from chromosomes 6q and 16q or expression of the c-erb B-2 oncogene. Loss of heterozygosity in chromosome 22 is the most frequent event in meningiomas, suggesting the presence of a tumor-suppressor gene in this chromosome.