Preoperative evaluation of ovarian masses has become increasingly important for optimal planning of treatment. The aim of this study was to assess the role of preoperative serum cancer antigen 125 (CA-125) levels in correlation with ultrasonographic features in order to distinguish between borderline ovarian tumors (BOTs) and stage I epithelial ovarian carcinoma (EOC). Retrospective study. Tertiary University Hospital. We reviewed all women with BOTs and stage I EOC from January 2000 to December 2010. Data from 165 women (66 BOTs and 99 stage I EOC) were analyzed. Multivariable logistic regression with stepwise selection of variables was used to determine which clinical variables, ultrasound features and CA-125 level were independently associated with invasiveness. Utility of ultrasonographic markers and CA-125 in the preoperative differential diagnosis between BOTs and stage I EOC. Women with CA-125 >100IUmL(-1) had almost three times greater likelihood of belonging in the EOC group [odds ratio (OR) 3.02; confidence interval (CI) 95%: 1.13-8.12]. Furthermore, the presence of large solid component (≥20% of the tumor comprised of solid components) was associated with 4.25 times greater odds of it to representing ovarian cancer rather than a BOT (OR 4.25; 95% CI: 2.05-8.82). In contrast, the presence of papillary projections was associated with a 73% lower likelihood of EOC (OR 0.27; 95% CI: 0.13-0.58). Preoperative CA-125 >100IUmL(-1) combined with the presence of a large solid component and the absence of papillary projections seems to improve the discriminative ability in favor of stage I EOC.