Ovarian cancer is the fifth leading cause of cancer death in women. Absence of a reliable biomarker precludes early diagnosis of the disease. To identify new proteins with potential diagnostic or prognostic value for the therapy of ovarian cancer we performed comparative proteomic analysis of sera from ovarian cancer patients and healthy women. We analyzed serum samples from 10 patients diagnosed with epithelial ovarian cancer and 10 age-matched healthy women. To decrease the extremely wide dynamic range of protein concentrations in serum we used combinatorial hexapeptide libraries. Serum samples were then subjected to proteomic 2-DE analysis. Three proteins with differential abundance were found and identified by mass spectrometry: α-1-antitrypsin, apolipoprotein A-IV and retinol-binding protein 4. Identification of α-1-antitrypsin and apolipoprotein A-IV confirms previous studies but the identification of significantly decreased levels of RBP4 in ovarian cancer patients represents a novel observation. We verified the decrease of RBP4 levels in ovarian cancer patient sera by two independent methods and determined absolute RBP4 concentrations in patients and healthy women. We excluded possible non-cancer factors that could be responsible for the observed RBP4 decrease. We propose a connection of RBP4 with epithelial ovarian cancer and advocate the potential of RBP4 as a candidate diagnostic or prognostic biomarker.