GPER-1 protein expression was immunohistochemically examined in 164 primary breast cancer specimens and their matched normal breast epithelium. GPER-1 down-regulation correlated significantly with increased histological grading (p = .015), lymph node metastases (p = .032), and negative estrogen receptor status (p = .018). The decrease of GPER-1 expression in breast cancer tissue, relative to normal tissue, was associated with poor overall survival (p = .043) and disease-free survival (p = .037) and remained a significant unfavorable factor in multivariate analysis for DFS (HR = 1.569; 95% CI, 1.024-2.797; p = .041) and OS (HR = 2.082; 95% CI, 1.248-4.773; p = .039). Thus aberrant GPER-1 expression seems to be an important factor in breast cancer progression.