Background: An External Quality Assessment (EQA) program was developed to investigate the state of the art of HER2 immunohistochemical determination in breast cancer (BC) in 16 Pathology Departments in the Lazio Region (Italy). This program was implemented through two specific steps to evaluate HER2 staining (step 1) and interpretation (step 2) reproducibility among participants. Methods. The management activities of this EQA program were assigned to the Coordinating Center (CC), the Revising Centers (RCs) and the Participating Centers (PCs). In step 1, 4 BC sections, selected by RCs, were stained by each PC using their own procedures. In step 2, each PC interpreted HER2 score in 10 BC sections stained by the CC. The concordance pattern was evaluated by using the kappa category-specific statistic and/or the weighted kappa statistic with the corresponding 95% Jackknife confidence interval. Results: In step 1, a substantial/almost perfect agreement was reached between the PCs for scores 0 and 3+ whereas a moderate and fair agreement was observed for scores 1+ and 2+, respectively.In step 2, a fully satisfactory agreement was observed for 6 out of the 16 PCs and a quite satisfactory agreement was obtained for the remaining 10 PCs. Conclusions: Our findings highlight that in the whole HER2 evaluation process the two intermediate categories, scores 1+ and 2+, are less reproducible than scores 0 and 3+. These findings are relevant in clinical practice where the choice of treatment is based on HER2 positivity, suggesting the need to share evaluation procedures within laboratories and implement educational programs. © 2013 Terrenato et al.; licensee BioMed Central Ltd.