Background: It is well known that there is considerable inter-observer variability in assessment of the pathological parameters that are used to select node-negative breast cancer patients for adjuvant systemic treatment. There are only limited data available as to in how many patients this leads to differences in treatment decisions. Methods: Clinical and pathological data of 694 patients <61 years with primary unilateral T1-4N0M0 breast cancer were analysed. Grade, estrogen receptor (ER) status and human epidermal growth factor receptor 2 (HER2) status were first assessed locally; subsequent central re-evaluation of these parameters was carried out. Clinicopathological low or high risk was assessed using national Dutch guidelines and the Adjuvant! Online (www.adjuvantonline.com). Results: The local pathological examination was discordant with central review for grade, ER and HER2 in 28% (kappa 0.56; grade 2 tumours 35% discordant), 5% (kappa 0.85) and 4% (kappa 0.81) of patients, respectively. If clinical risk were assessed based on Dutch guidelines or Adjuvant! Online, respectively, 15% (one of seven patients; kappa 0.70) or 8% (kappa 0.83) of patients would have been assigned to a different clinical risk group. Conclusion: Inter-observer variation in pathological examination of breast carcinomas results in significant differences in grade, ER status, HER2 status, clinicopathological risk and subsequently in adjuvant systemic treatment advice.