Malignant gliomas and medulloblastomas which are the most common primary malignant brain tumours of adults and children, respectively, resemble other neurogenic tumours as they frequently contain gene amplification and show non‐random loss of specific chromosomal regions. In gliomas the gene which is most often amplified, is the epidermal growth factor receptor gene. In many cases the gene is rearranged as well, producing abnormally small epidermal growth factor receptor proteins. More than 80% of tumours have lost chromosome 10 and losses of 9p13, 17p and 22 occur in subgroups of cases. 17p loss is associated with point mutations of the p53 gene, but the relevant genes in the other chromosomal regions remain to be identified. For medulloblastoma the most frequent chromosomal abnormality is i (17q). Whether or not p53 gene mutations are the targets of 17p losses in these tumours remains to be determined. Approximately 5% of medulloblastoma biopsies contain gene amplification, although the incidence in medulloblastoma cell lines is more than 80%. c‐mycis the gene which is most frequently amplified in this tumour type. The relationship of these various molecular genetic abnormalities to the biology of the tumours and the course of the patients remains largely unexplo