Immunohistochemical (IHC) assays are routinely used for determining the estrogen receptor (ER) and progesterone receptor (PgR) status of women with breast cancer. The present knowledge of reproducibility of the subjective IHC assessment is limited. The purpose of this study was to evaluate the interobserver reproducibility in IHC assessment of 127 (ER) and 126 (PgR) cases of primary breast cancer. The slides were independently re-assessed by 22 pathologists from nine hospitals. A simple assessment protocol was used with a cut-off value > 10% stained nuclei, regardless of staining intensity. The concordance was high, with a kappa value of 0.78 for ER and 0.72 for PgR. Since some of the 22 pathologists considered one or several sections to be inevaluable, the evaluation comprised three groups (< or = 10% vs. > 10% vs. not evaluated). Although the percentage of positive cases varied among the 22 pathologists (ER: 57%-79%; PgR: 41%-66%), the overall concordance was good for both ER and PgR (kappa = 0.78 and 0.72, respectively). A further improvement in concordance should be possible to achieve through training.