Amplification of the gene for epidermal growth factor receptor (EGFR) is a common finding in malignant gliomas. We found that 18 of 29 grade 3 and grade 4 gliomas had EGFR amplification when assayed using fluorescence in situ hybridization. The amplification pattern suggests that the amplicon is contained within double minute chromosomes in most cases. EGFR copy number can differ by 20-fold in amplified cells within a single case. Polysomy 7 occurs frequently in both EGFR-amplified and -unamplified cells. More than one-third of the cases had < or = 10 percent of cells with amplified EGFR, and it is likely that these cases would not have been identified by methods that do not examine DNA on a cell by cell basis.