Background: The treatment guidelines in the last decade have shown a trend towards increasing surgical radicality in endometrial cancer. Little information is available on the implementation of standards into clinical reality. We evaluated the adherence to standard therapy before and after introduction of an internal quality management system and determined the reasons for non-adherence. Patients and Methods: A retrospective analysis of the inhouse tumor registry was performed. Included were all patients with Federation of Gynecology and Obstetrics (FIGO) I–III endometrial cancer and therapy at the Dr. Horst Schmidt Klinik (HSK) from 1997 to 2007. Results: 206 patients with epithelial endometrial cancer in stage FIGO I–III underwent primary surgery at the HSK. 140 (68%) patients were operated as recommended by the guidelines. 20% of patients were operated less radically (17% vs. 22% before and after introduction of guidelines; p = 0.33) and 12% more radically. The latter was significantly reduced after implementation of quality management (21% vs. 7%; p = 0.004). Comorbidities and age played an important role in less-than-standard treatment. Conclusions: Adherence to guideline-based therapy for endometrial cancer can be achieved in most patients. Implementation of standards and quality assurance primarily helps to avoid surgical overtreatment but failed to reduce less-than-standard treatment radicality. The latter seemed to be more defined by patient characteristics than by institution standards.