HER2 (human epidermal growth factor receptor 2) status became an important prognostic and predictive factor in breast carcinoma clinical management. There are two main techniques of evaluation of HER2 status: immunohistochemistry (IHC) for the protein expression and fluorescence in situ hybridization (FISH) for amplification of HER2 gene. The aim of the study was to compare the results obtained by IHC and FISH methods in determination of HER2 status in breast cancer. Three hundred and sixty breast cancer specimens were examined. Patients were operated in the Oncology Centre in Warsaw. IHC and FISH were performed in every case. IHC was performed with DAKO HercepTest and FISH with Oncor-QBiogene reagents. IHC results were classed into 4 groups, accordingly to the four-tier DAKO criteria system (0, 1+, 2+, 3+). FISH results were divided into three main categories: NA--no amplification, LA--low amplification and HA--high amplification. The number of copies of chromosome 17 was also assessed. Over 90% of cases described by IHC as 3+ exhibited amplification of HER2/neu gene. Remaining cases were positive with IHC, but presented no gene amplification. This might be due to the subjective assessment of the membrane staining. Another possibility is that overexpression of the protein was caused by mRNA stability or disorders in receptor degradation. The majority of cases classed by IHC as 2+ were also negative by FISH (80%). One fifth of IHC 2+ tumours were found to exhibit gene amplification. Remaining cases showed no amplification of HER2/neu gene, combined with aneuploidy of chromosome 17. All cases described by IHC as 0/1+ were also HER2-negative by FISH. IHC is well-established method of assessing HER2 status in breast cancer. Nonetheless, a group of cases described as 2+ should be additionally examined using FISH. The results obtained by the latter method are more reliable. In order to improve accuracy and gain the highest quality of HER2 status evaluation, in 2+ cases both methods should be applied.