Assessing hormone receptor status is an essential part of the breast cancer diagnosis, as this biomarker greatly predicts response to hormonal treatment strategies. As such, hormone receptor testing laboratories are strongly encouraged to participate in external quality control schemes to achieve optimization of their immunohistochemical assays. Nine Dutch pathology departments provided tissue blocks containing invasive breast cancers which were all previously tested for estrogen receptor and/or progesterone receptor expression during routine practice. From these tissue blocks, tissue microarrays were constructed and tested for hormone receptor expression. When a discordant result was found between the local and TMA result, the original testing slide was revised and staining was repeated on a whole-tissue block. Sensitivity and specificity of individual laboratories for testing estrogen receptor expression were high, with an overall sensitivity of 99.7 and 95.4 %, respectively. Overall sensitivity and specificity of progesterone receptor testing were 94.8 and 92.6 %, respectively. Out of 96 discordant cases, 36 cases would have been concordant if the recommended cut-off value of 1 % instead of 10 % was followed. Overall sensitivity and specificity of estrogen and progesterone receptor testing were high among participating laboratories. Continued enrollment of laboratories into quality control schemes is essential for achieving and maintaining the highest standard of care for breast cancer patients.