Worldwide, more than 6,044,000 women are diagnosed with cancer each year and more than 3,345,000 lose their battle with it. About 12% of all cases are attributed to malignant diseases of gynecological origin. In turn, ovarian tumors account for 4% of cancers in women with more than 190,000 cases diagnosed each year. About 85% of cases are in women over 50 years of age, and the most common morphological feature is of epithelial origin. Most cases of ovarian tumor are detected at an advanced stage and the prognosis is accordingly poor. Therefore, the task of finding markers whose presence corresponds to an increased risk of developing this type of disease is important. Tumor markers are molecules that are produced in response to neoplastic proliferation and then enter the systemic circulation. They provide information about the possible presence of cancer or its behavior and evolution. In the context of tumor marker interpretation, one must initially assess its sensitivity (number of cases detected after a positive result) and specificity (number of non-cancer cases detected after a negative result). An ideal tumor marker would have 100% sensitivity and 100% specificity, but in practice this is not achieved. For ovarian neoplasia, the tumor marker that is used is called CA 125. It is not specific for ovarian cancer, but is spread in various tissues. It is produced by structures derived from the coelom epithelium such as the cervical canal, endometrium, and fallopian tubes. Other epithelial-derived organs that also produce CA 125 are the colon, pancreas, lungs, mammary glands, and stomach. It is also found in tissues such as the pleura, pericardium, and peritoneum. NEWS_MORE_BOX The widely accepted reference limit of 35 kU/L results from the average level of the tumor marker in healthy subjects. However, it can be measured within wide limits due to the fact that its level is affected by age, race, menstrual cycle, presence of pregnancy or many benign conditions. For example, in postmenopausal women, the CA 125 level, reaching 20 kU/L, is usually lower than that of the general population. Other factors that lower levels of the marker include black race, caffeine intake, and hysterectomy (removal of the uterus). On the other hand, conditions such as endometriosis, fibroids, infections, and pelvic inflammatory disease raise CA 125. Pregnancy is a physiological state in which CA 125 rises, with the high level remaining for up to 10 weeks postpartum, and levels can reach 120 kU/L. Since the tumor marker is also found in other structures, it can increase in inflammation of the peritoneum, pleura, pancreas or in oncological diseases of the mammary glands, lungs, colon and others. About 85% of women with ovarian tumors of epithelial origin have elevated CA 125 levels,and in the advanced stages it increases to 90%. CA 125 is less often increased in mucinous or borderline tumors. Also, a single increased level of a tumor marker is not fatal. A tumor marker is not a diagnosis, but a tool that helps assess the risk of a certain disease being present.