Purpose: The aim of this study was to investigate the relationship between ovarian volume and serum CA-125 levels. Materials and Methods: Serum CA-125 levels and ovarian volume were compared among the cases with benign ovarian neoplasms, primary epithelial ovarian cancer (EOC), controlled ovarian hyperstimulation, and ovarian hyperstimulation syndrome (OHSS). Also, the correlation between CA-125 levels and ovarian volume were evaluated in the presence of peritoneal fluid and/or peritoneal carcinomatosis. Results: Although ovarian volume was not different among the groups, CA-125 levels were higher in the cases with EOC than with benign ovarian tumors (p = 0.001). Baseline CA-125 levels were not found to have increased while ovarian volume went up with controlled hyperstimulation in the infertile group (p = 0.555). However, uncontrolled hyperstimulation of the ovaries and the presence of peritoneal fluid caused an increase in the levels of CA-125 (p = 0.001). There was no correlation between ovarian volume and CA-125 levels in the cases with malignant ovarian tumors (r = 0.083). Conclusions: The results of this study have confirmed that CA-125 is a peritoneal marker and increased ovarian volume with benign ovarian neoplasms or controlled hyperstimulation does not increase CA-125 levels in the same way. The presence of peritoneal carcinomatosis and/or peritoneal fluid seems to be an important factor for high CA-125 levels in patients with epithelial ovarian cancer (EOC).