Glioblastoma (GBM) is the most common primary brain tumor in adults and often has amplification of the epidermal growth factor receptor (EGFR) gene. The value of EGFR as a prognostic marker in GBMs is unclear; some studies have shown an adverse correlation, whereas others have indicated a neutral or even favorable association with longer survival. Furthermore, EGFR-amplified GBMs are usually regarded as a single subgroup of tumors, although the range of EGFR copy number varies greatly. In this study, 532 GBMs were analyzed for EGFR amplification via fluorescence in situ hybridization at the time of initial diagnosis. Although there was no difference in survival by EGFR amplification (P = 0.33), stratification by the amount of EGFR amplification showed that, surprisingly, median survival was 39% longer in the high-amplifier group (EGFR:chromosome 7 ratio >20) compared to nonamplified GBMs (P = 0.03) and was 43% longer compared to GBMs with low to moderate EGFR amplification (EGFR:chromosome 7 ratio = 2 to 20; P = 0.0007). Stratifying by postsurgical treatment regimens, this difference was seen only when temozolomide (TMZ) was used; tumors without amplification and with high EGFR amplification both responded better to TMZ than those with low to moderate amplification (P = 0.01), whereas GBMs that had not been treated with adjuvant therapy nor with adjuvant therapy lacking TMZ showed no survival differences (P = 0.63 and 0.91, respectively). These results suggest that GBMs with EGFR amplification are a heterogenous group of tumors and that behavior might differ according to the degree of amplification, although not in a straightforward dose-response manner.