Introduction: In this study, we aimed to determine serum adenosine deaminase (ADA) and cancer antigen 125 (CA 125) levels in patients with epithelial ovarian tumors. Methods: A total of 75 patients with 47 malignant and 28 benign epithelial ovarian tumors were included in this case-control study. On the morning of the operation, serum CA 125 and ADA levels were determined. Postoperative epithelial ovarian tumor patients were divided into two subgroups as benign and malignant histopathologically. We evaluated whether there was a correlation between tumor levels and serum CA-125 and ADA levels of subgroups and malignant patients. Results: Of 47 patients with malignant epithelial ovarian cancer, 20 of them had an early-stage (stage 1-2) and 27 of them had advanced-stage (stage 3-4) tumors. Serum CA-125 (26.38 vs 44.93, p<0.001) and ADA levels (29.91 vs 42. 82, p≤0.05) were significantly higher in malignant epithelial ovarian tumors than in benign epithelial ovarian tumors. There was no significant correlation between ADA levels. Discussion and Conclusion: This study showed that serum ADA levels in patients with epithelial ovarian cancer could be used as a biomarker in combination with other parameters for predicting malignancy in ovarian cancer. To validate these clinical data, large-scale studies of epithelial ovarian tumors are needed.