Objective: The aim of this study was to determine the relationship between the levels of tumour marker CA 125 antigen and pelvic tumour size, histopathological type, stage, bilateral status, ascites, type of surgery, and postoperative complications. Materials and Methods: A retrospective cross-sectional descriptive study was conducted on 203 patients with a pelvic mass who were visited in the Shahid Sadoughi hospital in Yazd, Iran from 2007 to 2010. Data were analyzed by software SPSS v.14. Results: Statistical analysis, based on Fisher’s exact test, showed that patients with pelvic mass who presented with either of bilateral involvement/ ascites (p = 0.000), higher stage (p = 0.001), inability for complete resection (p = 0.000), or postoperative complications (p = 0.001) had significantly higher serum concentrations of CA 125 antigen. There was no relationship between serum level of CA 125 and such variables as tumor size (p = 0.883) and abdominal ultrasound findings (p = 0.297). Conclusion: Using CA 125 as a diagnostic and prognostic tool in patients with newly-discovered pelvic mass can be helpful in some aspects, but cannot estimate size of the tumor and its solid/cystic status. It also cannot predict post-surgical complications of malignant pelvic masses.