Objectives: The global obesity epidemic has great impact on the prevalence of low-grade endometrial carcinoma. The preoperative tumor serum marker cancer antigen 125 (CA-125) might contribute to improved identification of high-risk patients within this group. The study aimed to investigate the prognostic value of CA-125 in relation to established preoperative prognosticators, with a focus on identifying patients with poor outcome in low-grade endometrial carcinoma (EC) patients. Methods: Prospective multicenter cohort study including all consecutive patients surgically treated for endometrial carcinoma in nine collaborating hospitals from September 2011 until December 2013. All preoperative histopathological diagnoses were reviewed in a blinded manner. Associations between CA-125 and clinicopathological features were determined. Univariable and multivariable analysis by Cox regression were used. Separate analyses were performed for preoperatively designated low-grade and high-grade endometrial carcinoma patients. Results: A total of 333 patients were analyzed. CA-125 was associated with poor prognostic features including advanced International Federation of Gynecology and Obstetrics (FIGO) stage. In multivariable analysis, age, preoperative tumor and CA-125 were significantly associated with disease-free survival (DFS); preoperative grade, tumor type, FIGO and CA-125 were significantly associated with disease-specific survival (DSS). Low-grade EC patients with elevated CA-125 revealed a DFS of 80.6% and DSS of 87.1%, compared to 92.1% and 97.2% in low-grade EC patients with normal CA-125. Conclusion: Preoperative elevated CA-125 was associated with poor prognostic features and independently associated with DFS and DSS. Particularly patients with low-grade EC and elevated CA-125 represent a group with poor outcome and should be considered as high-risk endometrial carcinoma.