Most Her2 testing guidelines recommend that all cases scoring Her2 2+ by immunohistochemistry should be analyzed by fluorescent in situ hybridization (FISH) to determine HER2 status to confirm eligibility for Trastuzumab therapy in breast cancer. The aim of our study was to determine HER2 gene and chromosome 17 (CEN17) status in a series of 108 Her2 2+ consecutive cases and study the correlation between pathological characteristics of the tumors and HER2 amplification. Invasive breast cancers were tested by FISH using the Dako HER2 FISH pharmDx kit. The Her2 immunohistochemistry protocol was performed using the polyclonal AO485 antibody (Dako) diluted to 1:1500. HER2 and CEN17 status were correlated to tumor SBR grade, mitotic count, estrogen receptor, progesterone receptor status and percentage of Her2 immunohistochemistry-positive cells. Following Food and Drug Administration guidelines, ie, HER2/CEN17 ratio >or=2 and an HER2 copy number >4, amplified cases were observed in 36 (33%) and 49 (45%) cases, respectively, and following American Society of Clinical Oncology/College of American Pathologists guidelines, ie, HER2/CEN17 ratio >2.2 and an HER2 copy number >6, amplified cases represented 30 and 24% of the study population, respectively. Chromosome 17 polysomy (CEN17 >2.25) was observed in 39 (36%) tumors. Significant positive correlations were found between FISH HER2 amplified cases and Her2 immunostaining >60% (P=1.1.10(-5)), SBR grade 3 (P=0.0001), nuclear atypia (P=0.03) and mitotic count (P=0.008). By multivariate analysis, Her2 immunostaining >60% (P<10(-3)) and SBR grade 3 (P<10(-3)) were independent factors predicting HER2 amplification status irrespective to cutoff guidelines. All SBR grade 3 cases with more than 60% Her2+ cells had an HER2/CEN17 ratio >or=2, only one had a ratio <or=2.2. In our series of consecutive Her2 2+ cases, one-third demonstrated HER2 amplification, and one-third had chromosome 17 polysomy. Pathological factors, in particular SBR grade 3 and more than 60% Her2+ cells, were significantly correlated with HER2 amplification.