HER2 gene amplification in invasive breast cancer is a robust predictive marker for response to transtuzumab therapy. This study was undertaken to measure concordance between immunohistochemistry (IHC) and FISH for HER2 gene amplification in invasive breast tumors, as well as the presence of polysomy 17 and possible correlation with demographics and histopathological variables, including ER and PR positivity. A total of 425 cases of infiltrating carcinoma of breast (99% IDC-NOS) were studied. HER2 over expression was tested by IHC and FISH methods. Association between IHC and FISH in both subsets was calculated by amplification ratio including polysomy 17. Out of 425 specimens, 128 (30%) were positive for HER2 amplification by FISH test, whereas only 78 (24%) tumors with 2+ expression showed amplification. In contrast, 39 (74%) demonstrated 3+ IHC score and HER2 gene amplification. The histological variables including tumor size, tumor type, and lymph node involvement did not influence the outcome of FISH analysis. The ER and PR status showed significantly greater positivity in Patients negative for HER2 amplification. Polysomy 17 was detected in 23.7% Patients and was positively associated with ER and PR expression (P=