OBJECTIVES: Breast cancer (BC) is the most common malignancy in women. Treatment guidelines recommend specific diagnostic measures and a subsequent treatment. The aim of this study was to investigate adherence to guidelines regarding diagnostics and treatment of early BC patients, and the association of guideline adherence with overall survival during the first year after diagnosis. METHODS: All patients with an incident BC diagnosis (24 months wash-out period; ICD-10: C50) without metastases (early BC) between 01/01/2012 and 30/03/2018 were included from a German claims dataset (AOK PLUS). Recommendations regarding diagnostics and treatment from relevant German guidelines were extracted; subsequently, guideline adherence was assessed in each patient. Impact of guideline adherence on the risk for early death was ascertained using a multivariable Cox regression model. RESULTS: Of 13,671 patients with early BC, 98.4% were female. Mean age was 69.0 years (SD 15.2) at first observed BC diagnosis, mean Charlson-Comorbidity-Index (CCI) was 3.6 (SD 2.4). Within 1 year before or 90 days after initial diagnosis, 73.0% of the patients underwent mammography, 52.7% sonography, in 57.5% a biopsy was done and 21.0% did not receive any of these. One third of all patients did not receive any treatment in the first year (32.2%), whereas 67.8% received a treatment: 58.2% had a surgery, 43.9% radiotherapy and 59.0% received any pharmacological treatment. The risk to die within the first year after first BC diagnosis was higher for patients without any diagnostics (HR 2.792; p<0.001) and without any treatment (HR 2.085; p<0.001), talking age and CCI of patients into account. These results were checked and confirmed by excluding all patients that died within the first 90 days (no diagnostics: HR 2.354; p<0.001; no treatment: 1.588; p<0.001). CONCLUSIONS: A substantial percentage of early BC patients are not treated according to guidelines. Guidline non-adherence is associated with a higher mortality risk.