Abstract:To observe the level of extracellular domain (ECD) of human epidermal growth factor receptor-2 (HER-2neu) in patients with breast cancer, and to investigate its relationship with the clinical characteristics of breast cancer and the expression of HER-2neu in breast cancer tissues. Methods Fifty patients with breast cancer were selected as the breast cancer group and 50 cases of healthy volunteers were selected as the control group in Zhejiang Cancer Hospital from January 2013 to December 2015. The serum level of HER-2neu ECD was measured by chemiluminescence. The expression of HER-2 protein in breast cancer tissues was detected by immunohistochemistry. Results The serum level of HER-2neu ECD in the breast cancer group was significantly higher than that in the control group ( p< 0.05). The positive rate of serum HER-2neu ECD in the breast cancer group was higher than that in the control group ( p< 0.05). In the 50 cases of breast cancer tissue specimens, 20 cases had positive expression of HER-2neu, the positive rate was 40.0%. The positive rates of HER-2neu ECD in the patients with TNM stage Ⅲ, negative ER,negative PR and tumor ≧3 cm were higher than those in the patients with TNM stage Ⅰand Ⅱ, positive ER, positive PR and tumor <3 cm (p < 0.05). Of the 20 breast cancer patients with positive HER-2 in the breast cancer tissues, 9 patients were positive for serum HER-2neu ECD. Of the 30 breast cancer patients with negative HER-2 in breast cancer tissues, 29 cases were negative for serum HER-2neu ECD. The two methods were consistent (Kappa = 0.455, = 0.000). Conclusions The serum level of HER-2neu ECD in patients with breast cancer is increased, and it is consistent with the expression of HER-2 protein in breast cancer tissues. The serum level of HER-2neu ECD is associated with TNM stage, ER, PR and tumor size, therefore it can be used as the supplementary method for detection of HER-2 protein expression in breast cancer tissues.