Objective: This study aims to develop a nomogram to predict fluorescence in situ hybridization(FISH) test results for immunohistochemistry (IHC)-determined HER2 borderline breast cancer patients in China.Method: We reviewed an electronic database of breast cancer patients diagnosed between January 2007 and April 2013 at our institutions.Patients with IHC-determined HER2 negative and positive diseases were included as the training cohort, whereas those with IHC-determined HER2 borderline (2+) diseases were identified as the validation cohort.We used logistic regression to develop a nomogram, and we used receiver operating curve analysis and calibration plots to validate our nomogram.Result: In total, 1138,301 and 344 patients had IHC-determined HER2 negative, borderline and positive diseases, respectively.Within the training cohort, univariate and multivariate analyses suggested that estrogen receptor (ER), progesterone receptor (PR) and grade were significantly associated with HER2 status (P<0.00).A nomogramwas developed,and the AUC values were 0.795 and 0.749 in the training and validation cohorts, respectively.The calibration plots suggested that the model was well-calibrated.Conclusion: The new nomogram predicts HER2 status in IHC-determined HER2 borderline breast cancer patients.Our nomogram will be helpful, particularly for resource-limited countries.More studies are needed to confirm the accuracy of the nomogram.