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Background: The clinicopathological characteristics of bilateral breast cancer (BBC) in Japanese women have not been cleared and their impact on the survival has been still conflicting.However, it is known that Asian patients had a slightly greater risk of developing any second primary breast cancer compared with non-Hispanic whites, precise Japanese BBC data has not been published.Purpose: The objective of this study is to survey the incidence of BBC and address the clinicopathological difference compared to unilateral breast cancer (UBC) in Japanese women.Material and Method: 947 patients treated between 2005 and 2008 were included in this study.All patients were women and seventy six (8.0%) were BBC patients.These BBC patients were categorized as either synchronous BBC patients (bilateral breast cancers were diagnosed at same time or the interval was within 12 months) or metachronous BBC patients (contralateral breast cancer was diagnosed after 12 months of the first diagnosis).We compared the differences of age at diagnosis, clinical stage, family history of breast and/or ovarian cancer, lymph node status, histopathology, hormone receptor status, and disease free survival.In metachronous BBC, the data of the later one was adopted.Result: Twenty nine synchronous BBC patients and forty seven metachronous BBC patients were included.The median age of diagnosis of UBC cases was 55 years old (28-89), and that of BBC cases was 54 years old (32-91).Family history of breast and/or ovarian cancer has proven to be a significant risk factor of BBC (p=0.043).As for the histopathological features, although 10% of UBC was ductal carcinoma in situ (DCIS), the rate of DCIS of BBC was 26% (p<0.001).The HER2-positive rate was only 15% for the UBC cases, but only 4% for the BBC cases (P 3f=3f0.006).Moreover the incidence of triple negative breast cancer in BBC was significantly higher than that of UBC (p=0.024).Mastectomy was more common selection in women with BBC (p=0.002).The incidence of local recurrence rate of bilateral breast cancer patients was higher than that of unilateral breast cancer patients.There were no significant differences in hormone receptor status and age.Conclusion: According to our data, we found that Japanese BBC patients tend to have a family history of breast cancer and/or ovarian cancer.Also the histopathological findings were distinctive; the rate of triple negative cancer in BBC cases was much higher compared to that of UBC cases.The of BRCA1 as the causative gene of familial breast cancer and/or ovarian cancer is known to have high incidence of triple negative breast cancer.Our results also suggested that.