Background: To determine the amount of co-expression of IDO and EGFR in breast cancer patients. Materials and Methods:In order to obtain the distribution of co-expression of IDO and EGFR in breast cancer, we tested 110 breast cancer paraffin tissue blocks with immunohistochemical methods. Then we investigated the relationship between the diagnostic and pathologic characteristics (tumor size, lymph node status, histologic grade, the gene expression of ER, PR, HER2, p53, Ki67 and PCNA) with the situation of co-expression of IDO and EGFR by reviewing the medical records of 32 breast cancer patients. Results: Among 110 breast cancers, 32 cases demonstrated IDO and EGFR co-expression (29.1%), IDO and EGFR synchronous co-expression being found in 19.1% and asynchronous in 10.0%. Conclusions: IDO and EGFR were co-expressed in breast cancer, including synchronous and asynchronous co-expression. The results suggest that considering IDO and EGFR as two indicators for breast cancer treatment or prognosis analysis provides a potential option of individual treatment for the portion of breast cancer patients with co-expression of IDO and EGFR.