ObjectiveTo analyze the clinicopathologic features and prognosis of breast cancer patients with low human epidermal growth factor receptor-2 (HER2) expression. MethodsThe breast cancer patients underwent initially surgical resection in the First Hospital of Shanxi Medical University from October 2015 to October 2017 and met the criterion of this study were retrospectively gathered. Based on the immunohistochemical / in situ hybridization detection results, the patients were divided into three subtypes of HER2 zero, low, and positive expressions, and the differences in the clinicopathologic characteristics, overall survival (OS) and disease-free survival (DFS) of the three subtypes of breast cancer patients were compared. At the same time, the risk factors affecting the OS and DFS of breast cancer patients with low HER2 expression were analyzed. ResultsA total of 315 eligible patients were gathered in this study, including 68 patients with HER2 zero expression, 121 patients with low HER2 expression, and 126 patients with positive HER2 expression. There were no statistic differences in the menstrual status, T stage, and histological classification between the breast cancer patients with low HER2 and positive HER2 expressions (P>0.05), but the proportions of the patients with lymph node metastasis, histological grade Ⅲ, negative hormone receptor (HR) and high Ki67 expression in the low HER2 expression patients were lower than those in the positive HER2 expression patients. And compared with HER2 zero expression breast cancer patients, the proportions of premenopausal / perimenopausal, T2–4, N1–3, histological grade Ⅱ, ductal carcinoma, negative HR, and low Ki67 expression patients in the breast cancer patients with low HER2 expression were higher (P<0.05). While the survival curves of OS and DFS by Kaplan-Meier method had no statistic differences among the three subtypes of the breast cancer patients (χ2=0.070, P=0.966; χ2=0.362, P=0.835). The multivariate analysis results by Cox proportional hazards regression found that the low HER2 expression breast cancer patients with histological grade Ⅲ and negative HR had the higher risks of OS and DFS shortening (P<0.05). In addition, the risk of DFS shortening in the patients with T stage 2–4 and N stage 1–3 was increased (P<0.05). ConclusionsFrom the results of this study, breast cancer patients with low HER2 expression is different from the other two subtypes of breast cancer in terms of clinicopathologic characteristics. However, there are no statistical significances in comparing the OS and DFS of three types of breast cancer patients, but it is found that histological grading and HR are related to the OS and DFS of breast cancer patients with low HER2 expression, and it is also found that T stage and N stage are related to the DFS of breast cancer patients with low HER2 expression, so more attentions should be paid to the treatment plans.