Surgical staging in early-stage uterine cancer is controversial. Preoperative serum CA-125 may be of clinical value in predicting the presence of extra-uterine disease in patients with apparent early-stage endometrial cancer. Between October 6, 2005, and June 17, 2010, 760 patients were enrolled in an international, multicentre, prospective randomized trial (LACE) comparing laparotomy with laparoscopy in the management of endometrial cancer apparently confined to the uterus. Of these, 657 patients with endometrial adenocarcinoma had a preoperative serum CA-125 value recorded. Multiple cross-validation analysis was undertaken to correlate preoperative serum CA-125 with stage of disease (Stage I vs. Stage II+) after surgery. Patients' median preoperative serum CA-125 was 14 U/ml. A cutoff point of 30 U/ml was associated with the smallest misclassification error, and using this cutoff, 98 patients (14.9%) had elevated CA-125 levels. Of those, 36 (36.7%) had evidence of extra-uterine disease. Of the 116 patients (17.7%) with evidence of extra-uterine disease, 31.0% had an elevated CA-125 level. On univariate and multivariable logistic regression analysis, only preoperative CA-125 level, but no other preoperative clinical characteristics were found to be associated with extra-uterine spread of disease. Utilizing a cutoff point of 30 U/ml achieved a sensitivity, specificity, positive predictive value and negative predictive value of 31.0, 88.5, 36.7 and 85.7%, respectively. Elevated CA-125 above 30 U/ml in patients with apparent early-stage disease is a risk factor for the presence of extra-uterine disease and may assist clinicians in the management of patients with clinical Stage I endometrial cancer. ; James Nicklin, Monika Janda, Val Gebski, Thomas Jobling, Russell Land, Tom Manolitsas, Anthony McCartney, Marcelo Nascimento, Lewis Perrin, Jannah F. Baker, Andreas Obermair, and for the LACE Trial Investigators ; Martin Oehler is one of the doctors who recruited and treated patients in the LACE study