We compared a monoclonal antibody (SV2-61γ) and a polyclonal antibody (Dako 
HercepTest) in immunohistochemical assessments of human epidermal growth factor 
receptor 2 (HER2) expression in 73 samples of advanced gastric cancer. Results 
were scored as 0 to 3+, and equivocal or discordant (SV2-61γ/Dako HercepTest = 
0/2+, 0/3+, 1+/3+ or 2+/3+) cases were subjected to fluorescence in situ 
hybridization (FISH) analysis. The frequencies of HER2 scores of 2+ or 3+ were 
15.1% (11/73) using SV2-61γ and 38.4% (28/73) using Dako HercepTest. All of the 
equivocal or discordant cases with a HER2 score of 3+ using Dako HercepTest 
exhibited amplification of the HER2 gene regardless of the HER2 score determined 
with SV2-61γ. The results of the HER2 tests differed according to the antibodies 
used for immunohistochemistry that preceded FISH analysis, being 15.1% (11/73) 
using SV2-61γ and 23.3% (17/73) using Dako HercepTest. Thus, therapeutic 
decisions might be markedly influenced by the selection of antibody used in the 
HER2 test.