The aim of this study was to assess the frequency of egfr whole gene and CA intron repeat amplification in invasive breast cancer as a mechanism for epidermal growth factor receptor (EGFR) protein overexpression. By means of tissue microarrays, protein overexpression and whole gene amplification were assessed in 222 cases of invasive breast cancer by immunohistochemistry and FISH, respectively. First intron CA repeat amplification was assessed by Taqman RT-PCR. With FISH and RT-PCR, 4.7 and 6.3% of cases showed whole gene and first intron CA repeat amplification, respectively. Amplification dosage varied between two- and four-fold in RT-PCR. By immunohistochemistry, 17.3% showed EGFR overexpression. There was a low correlation between the different methods. In all, 2.9% of cases showed both whole gene amplification and intron CA repeat amplification, and 90.3% of cases were negative for both. Nearly 20% of cases with immunohistochemical protein overexpression showed intron CA repeat amplification, and only 2.2% of cases that were negative on immunohistochemistry showed such amplification. In all, 13% of cases with protein overexpression showed amplification by FISH, and only 1.6% of cases that were negative on immunohistochemistry showed such amplification. Of the cases with EGFR overexpression, 4 (25%) showed either whole gene or intron CA repeat amplification. In conclusion, whole gene amplifications of egfr are rare in invasive breast cancer and explain protein overexpression in only about 12.5% of invasive breast cancer cases. First intron first CA repeat amplification is another important mechanism for EGFR protein overexpression, explaining protein overexpression in about 18.7% of cases. However, since about 75% of cases with EGFR protein overexpression lack either of these amplifications, other expression regulating mechanisms must be considered.