Purpose of Investigation: The objective of the study was to evaluate cancer stem cell-related marker NANOG expression in ovarian serous tumors using Western blotting (WB) and to compare WB results to immunohistochemical (IHC) results of NANOG expression in the same tumors. Materials and Methods: Of the 37 ovarian tumor samples obtained intraoperatively, diagnosis of ovarian serous tumors was established histopathologically in 17 cases. WB and IHC for NANOG was performed on the parallel portions of the same ovarian tumors in the latter cases. The IHC staining samples were made up of a NANOG positive and a NANOG-negative group. Pursuant to summation of signal intensity and positive cell occurrence, the authors additionally divided the NANOG-positive group into three subgroups. Correlation coefficient between NANOG WB and NANOG IHC results was calculated. Results: NANOG measured by means of WB was significantly higher in the IHC determined NANOG-positive group than in the NANOG-negative group (p = 0.003). Comparison of the amount of NANOG measured by WB and IHC scores of individual cases revealed substantial dispersion of WB results among the NANOG subgroups; the dispersion was largest when NANOG was IHC only slightly-positive. In the NANOG moderate- and strongly-positive subgroups, WB values were higher and more homogenously arranged. In all IHC determined NANOG-negative cases NANOG WB values were low, with low value variability among tumor samples. However, correlation between NANOG WB results and NANOG IHC scoring subgroups revealed statistical significance (r = 0.73, p = 0.001). Conclusion: By means of WB and IHC the authors demonstrated NANOG to be a potential marker of ovarian high-grade serous carcinoma. Further research on the correlation between NANOG WB expression and clinical parameters is needed.