CA-125, which stands for cancer antigen 125, is a tumor marker that may be elevated in certain cancers. CA-125 is measured by a simple blood test. CA-125 is most associated with ovarian cancer, but it may also be elevated in other cancers, such as endometrial cancer, cancer of the fallopian tubes, lung cancer, breast cancer, and cancer of the gastrointestinal tract. It may also be elevated with a number of benign conditions, such as endometriosis, and may also be elevated during pregnancy. Thus, CA-125 is not specific for cancer. It is also not a sensitive test for cancer, since not every cancer will have an elevated CA-125.

CA-125 is particularly useful for ovarian cancer for following the response to treatment and predicting prognosis after treatment. Its role in screening for ovarian cancer is controversial and has not yet been adopted for widespread screening efforts in asymptomatic women, but it has some utility in aiding the diagnosis of a women who presents with symptoms suspicious for ovarian cancer.