Overexpression of human epidermal growth factor receptor 2 (HER2) is an important prognostic and predictive marker of response to anti-HER2 therapy in breast cancer. Our goal was to analyze the prognostic significance of moderate expression of HER2 in breast cancer with intermediate differentiation grade. We performed a multicenter retrospective register study of 8494 patients with primary non-metastatic breast cancer admitted between 2000 and 2011 to eight Clinics in Saxony-Anhalt, federal state of Germany. Patients were divided into three groups according to their HER2 score: 4073 were classified as HER2 negative (HER2 0 and 1+), 822 HER2 moderate (HER2 2+/HER2), and 1238 HER2 positive (HER2 3+ or HER2 2+/HER2+). HER2-positive cases were excluded from analysis. Tumors with moderate HER2 (HER2 2+) expression demonstrated an aggressive behavior and worse patient survival compared with HER2 0 and 1+ status. HER2 2+ status was associated with shorter median overall survival (OS) (P < 0.0001) in breast cancer patients with an intermediate grade of differentiation. Comparing low-grade and high-grade tumors, HER2 moderate expression did not significantly influence patient survival. In multivariate analysis after adjustment for other prognostic factors HER2 2+ status remained an unfavorable prognostic factor for OS (HR 1.224, 95 % CI 1.059-1.415, P = 0.006) in breast cancer patients with an intermediate grade of differentiation. HER2 2+ status is an unfavorable prognostic factor regarding the OS of breast cancer patients with intermediate grade of differentiation and could be used to identify patients, who may benefit from adjuvant therapy.