CA125 is well established as an accurate and reliable means of monitoring response to treatment and confirming relapse in ovarian cancer patients. Its role in follow-up after initial treatment is less certain and the subject of a current clinical trial. Measuring response with computerized tomography scans is futile in the majority of patients, as disease is often nonmeasurable at presentation, e.g., ascites or peritoneal deposits, or all measurable disease has been removed at the time of surgery. Serial changes in CA125 can be used as a reliable indicator of disease response or progression so that patients can be classified as responding or progressing according to either standard or CA125 criteria. These precise definitions are currently being prospectively validated in conjunction with the new response evaluation criteria in solid tumor response guidelines and are being incorporated into all future clinical trials.