Tissue expressed human epidermal growth factor receptor 2 (HER2) is an established parameter of breast cancer, and it is determined in routine clinical practice by histology. This study aims to assess the prognostic value of serum HER2 protein quantified using ELISA in 66 primary breast cancer patients. The median follow-up period was 94 months. Prognostic performance was assessed through receiver operating characteristic (ROC), Cox proportional hazards regression, and Kaplan-Meier analyses. There was a significant difference between serum HER2 levels of patients with and without recurrence. Prognostic associations were significant for age, grade, nodal status, follicle-stimulating hormone, oestradiol, and serum HER2. The cut-off point for serum HER2 levels was 6.0 ng/mL. Recurrence incidence was 29% in the serum HER2