Estrogen receptor (ER) and progesterone receptor (PR) are prognostic markers of breast cancer and predictive markers of response to endocrine therapy. To determine rates of ER and PR expression in invasive breast carcinoma among Korean women, the Breast Pathology Study Group of the Korean Society of Pathologists collected 1198 specimens of invasive breast carcinoma from 6 university hospitals. Immunohistochemical analysis was carried out using 1 antibody against PR and 3 antibodies against ER (1D5, 6F11, and SP1). Specimens were evaluated using the semiquantitative Allred score (scores >2 were considered positive). A total of 1077 cases were interpretable for all 3 anti-ER antibodies. ER expression was positive in 68.5% of cases using SP1, in 59.6% using 1D5, and in 58.9% using 6F11. Of 1073 interpretable cases, PR expression was positive in 51.7% of cases. The frequency distribution of Allred scores revealed a bimodal pattern (complete absence of staining or staining in most cells) for both ER and PR. Patients with discordant results for 2 different ER antibodies showed a median overall survival (between that of double-positive cancer and that of double-negative cancer). Our results showed that the rate of hormone receptor expression in breast carcinomas among Korean patients did not differ from that of western patients. In addition, SP1 was the most sensitive antibody for identifying ER expression in tumors. However, further evaluation is needed to determine which antibody is the best for selecting patients with discordant results who are likely to respond to endocrine therapy.