Aim of the study: To investigate the

differences and relevance, and to evaluate the clinical significance of fluorescence in situ hybridisation and immunohistochemistry (IHC) in de-tecting HER2 in gastric cancer tissues. Material and methods: The expres-sion of HER2 protein and the amplifi-cation of the HER2 gene in 118 gastric cancer tissues were detected by im-munohistochemistry and fluorescence in situ hybridisation, respectively. Results: Using IHC, we found that in 40 cases (33.9%) the HER2 expression was at level 0, in 33 cases (28.0%) the HER2 expression was at level 1+, in 16 cases (14%) the HER2 expression was at level 2+, and in 29 cases (25.6%) the HER2 expression was at level 3+, respectively. Using the FISH test, 38 of 118 cases (32.2%) were judged as positive results. The concordance rate between the results of IHC and FISH in all cases was 85.6%. The concordance rate of IHC and FISH was high in cases of HER2 expression at level 0, 1+ and 3+ according to IHC, but low in cases of 2+ according to IHC (43.8%). Conclusions: Immunohistochemistry cannot predict HER2 gene amplifica-tion accurately. The FISH test should be executed in IHC 2+ cases. Key words: immunohistochemistry, fluo-rescence in situ hybridisation, gastric cancer, human epidermal growth fac-tor receptor-2