Objective: We conducted a comprehensive concordance study of 6 ISH methods with Abbott FISH in a large series of breast carcinomas. Method: Tissue Micro Arrays (TMA) were constructed by taking 3 tissue-cores from praffin blocks of 402 primary breast carcinomas. Up to 384 cases were analyzable in 7 ISH assays. Scoring was performed by two independent observers without knowledge of the other ISH data according the ASCO-guidelines for HER2-testing. Cases were considered positive when the ratio was ≥2.0. Discordant cases were reviewed and scores were reassigned on consensus of opinion. Concordance and Cohen's kappa score were calculated in relation to FISH, Abbott. Results: In 372 cases analyzable with Abbott HER2 FISH, 12.1 % were HER2-positive. Concordances (kappa-scores) of the 6 other assays were: DAKO FISH 98.1 % (0.90), DAKO duoCISH 97.2 % (10.4 %), Zytovision FISH 99.1 % (0.96), Zytovision duoCISH 99.1 % (0.96), single probe SISH Ventana 98.9 % (0.95), dual probe SISH Ventana 99.4 % (0.97). Conclusion: Conclusion: Concordance of 6 HER2 ISH assays with Abbott FISH were shown to be 97.2 % or higher. In this study, DAKO assays had a lower kappa score with Abbott FISH than Ventana or Zytovision assays.