Most ovarian tumors contain ovarian cyst fluid (oCF) which can be easily obtained during surgery. This is the first study that explored if CA 125 in oCF could be of prognostic value for patients with epithelial ovarian cancer (EOC). Of 54 patients with primary EOC, oCF and preoperative serum were collected and clinicopathological data were retrospectively obtained. CA 125 was measured with the commercially available CA 125 assay. CA 125 in oCF (n=54, median: 55,500 U/ml, range: 590-10,200,000 U/ml) was always higher than in the corresponding serum (n=51, median: 179 U/ml, range: 13-11,000 U/ml) (p<0.001) and values were moderately correlated (R=0.337, p=0.016). CA 125 in oCF was associated with histology (p<0.001) and tumor grade (p=0.038). High levels of oCF CA 125 (>median) were significantly associated with a poor disease-free survival (DFS) (log-rank p=0.002 and p=0.005 univariate Cox-regression). Other factors associated with a poor DFS in univariate analysis were advanced FIGO stage, suboptimal debulking (both p<0.001), high tumor grade (p=0.025), serous histology (p=0.003) and high serum (>media) CA 125 (p=0.009). In multivariate analysis, only FIGO stage was of independent predictive value. These findings indicate that, although high levels of oCF CA 125 were significantly associated with a poor survival of EOC patients, CA 125 in oCF was not of independent predictive value and might therefore not be useful as a prognostic biomarker for EOC.