The most frequent cytogenetic alterations in primary brain tumors are losses of chromosomes or chromosomal regions and the presence of double minute chromosomes (dmins). The regions which are lost and the genes which are amplified are distinctive for individual tumor types. Most malignant gliomas contain gains of chromosome 7 and losses of chromosome 10; losses of chromosome 22, 9p, and the sex chromosomes occur in subgroups of cases. The gene most frequently amplified in tumors with dmins is the epidermal growth factor receptor gene. Medulloblastomas have losses of 17p and most cases with dmins have c-myc gene amplification. Meningiomas have losses or deletions of chromosome 22. Identification of these specific cytogenetic abnormalities in human brain tumors has provided the framework for identifying genes which are amplified in them and has identified chromosomal regions likely to contain tumor suppressor genes, the loss or inactivation of which is important in the development of these tumors.