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e11007 Background: Change in Ki-67 levels is a predictive marker of breast cancer, and it has been widely investigated in patients treated with preoperative endocrine therapy. Several studies have compared Ki-67 expression in untreated specimens obtained by core needle biopsy (CNB) and surgery. We conducted a retrospective study to assess Ki-67 expression between these two specimen types. Methods: 307 patients underwent primary operations for early breast cancer at Tokyo-West Tokushukai Hospital from August 2008 to October 2011. Among them, 193 patients who underwent CNB prior to the operation were enrolled. Patients with ductal carcinoma in situ and those undergoing neoadjuvant chemotherapy were excluded. A cutoff value of 20% was used as the Ki-67 positive criterion. We also examined ER, PgR, and HER2 expression and compared it with Ki-67 expression. Statistical significance for concordance rates between the two types of specimens was evaluated by Wilcoxon t-test. To evaluate the consequences of formalin and genetic heterogeneity, parameters such as operative method and tumor size were analyzed using chi-square test. We reexamined Ki-67 expression in the central laboratory among the Ki-67 discordant group to evaluate the discordance between the pathologist’s judgments. Results: The Ki-67 expression concordance rate between the two types of specimens was 77.7%, which was significantly lower than that for ER, PgR, and HER2 expression at 94.8%, 88%, and 91%, respectively. No significant differences in the operative method (mastectomy vs. breast conserving surgery) or tumor size (≤2cm vs. >2cm) were observed between the groups. We reexamined Ki-67 expression in the Ki-67 discordant group and found that the Ki-67 concordance rate had improved to 93.8%. Conclusions: The Ki-67 expression concordance rate between the two types of specimens was significantly lower than that of ER, PgR, and HER2 expression. However, a reexamination of Ki-67 expression in the central laboratory revealed no significant difference, suggesting the need for a standard pathological assessment of Ki-67 expression to clinically use Ki-67 as a predictive marker.