Several types of epidermal growth factor receptor (EGFR) gene mutations have been reported in glioblastomas, and in nearly all cases the alterations have been reported in tumors with EGFR amplification. The objectives of this study were to determine the frequency and diversity of EGFR mutations in glioblastomas and to determine whether gene mutation is inevitably associated with increased EGFR gene dosage. To accomplish these aims, we sequenced cDNA products representing the entire EGFR coding region in 44 glioblastomas, half of which had EGFR amplification. Coding sequence alterations were identified in 17 of the tumors, and each of these cases had amplified EGFR. No mutations were identified in the 22 tumors without EGFR amplification. An additional 26 glioblastomas with EGFR amplification were then examined to establish more reliable frequencies for each type of mutation identified in the tumors for which the entire gene was sequenced. Transcripts associated with the most common mutation lacked coding sequence for amino acids 6-273 (67%). This mutation has been described extensively in the literature. Transcripts encoding receptors that would truncate at amino acid 958 and transcripts encoding receptors that would lack amino acids 521-603 were the next most common types of alteration. Each of these were observed in 15% of the tumors with EGFR amplification. Other mutations were observed at lower frequencies, but among these were three cases with missense mutations. Sixteen of the 48 tumors with EGFR amplification showed multiple types of EGFR mutations (33%), and in one case it was determined that multiple alterations had occurred in the same transcript. In total, these data are consistent with EGFR mutation being exclusively and frequently associated with EGFR amplification. Furthermore, the determination of multiple EGFR mutations within individual tumors suggests that glioblastomas with EGFR amplification have the capacity to produce a variety of functionally distinct EGFRs.