Amplification of the ERBB2 oncogene has recently received attention as a target for antibody-based therapies and as a predictor of response to adjuvant chemotherapy. Modification of treatment strategies based on ERBB2 status has led to further interest in the genetic alterations that accompany ERBB2 gene amplification or overexpression. In this study, chromosome alterations that are associated with ERBB2 amplification were defined by comparative genomic hybridization (CGH). Additionally, fluorescence in situ hybridization (FISH) was used to validate gene amplification, and protein expression was detected immunohistochemically. ERBB2-amplified tumors as detected by FISH, immunohistochemistry (IHC), or CGH had twice as many CGH-defined chromosomal alterations (means of 11.8, 11.0, and 12.7, respectively) as the nonamplified tumors (means of 6.8, 7.0, and 5.6, respectively). ERBB2 positivity correlated with the total number of genetic events. A wide spectrum of copy number gains and losses was seen by CGH in all of the tumors. An increased number of losses of 18q and gains of 20q was found in ERBB2-positive tumors. Other common aberrations for all of the tumors were copy number gains of 1q (58%), 8q (52%), 20q (30%), and losses of 18q (39%), 13q (39%), and 3p (33%). A high degree of concordance was observed among the three methods in 33 primary breast cancers. The concurrence for ERBB2 detection between FISH and IHC was 90%, between FISH and CGH was 82%, and between IHC and CGH was 84%. This study shows that breast tumors showing erbB2 overexpression or gene amplification are genetically distinct from erbB2-negative tumors. These differences may relate to the mechanisms underlying altered response to adjuvant therapies and may define the responsiveness to erbB2-directed immunotherapy.