Objective: As patients with increased human epidermal growth factor receptor (HER2) overexpression are more likely to benefit from trastuzumabtreatment, the accuracy of HER2 receptor status in breast cancer patients is significant for appropriate disease management. However, this assessmentis not harmonized and results may be highly variable between centers. The aim of this study was to investigate the degree of interlaboratory variabilityin the results of HER2 expression reported by 5 participating centers and to assess the concordance between these centers and a reference laboratory.Materials and Methods: A total of 30 breast cancer samples were tested and scored for HER2 expression using immunohistochemical method in5 centers from Turkey and in a reference laboratory from Netherlands (Academic Medical Center, Amsterdam). All the participating centers had anexperience of more than 10 years regarding the HER2 testing. The results were compared both among the centers and with the reference laboratory.Results: When the concordance of participating centers and the reference laboratory was evaluated regarding negative (0-1+), equivocal 2(+) andpositive 3(+) classification of HER2 immunostaining, the highest concordance was found in Center-A, and the lowest in Center-C (Kendall's tau-bconcordance coefficient 0.911 and 0.724, respectively). The concordance of the centers with reference laboratory was 80.0% both in equivocal andpositive samples, while it increased up to 91.8% in negative samples.Conclusions: This study showed that in general there is sufficiently good agreement between the reference laboratory and the participating centersfor immunohistochemical HER2 assessment.