The use of a tumor marker not only depends upon its sensitivity and specificity, but also upon its ability to influence decisions between alternative plans for patient management. Use of beta human chorionic gonadotropin (hCG) for monitoring gestational trophoblastic neoplasia has set the standard to which other assays must be compared. Beta hCG and alphafetoprotein have provided useful markers for ovarian germ cell tumors. Recently, a monoclonal antibody-based immunoassay for CA 125 antigen has been used to monitor the treatment of epithelial ovarian carcinomas. Persistent elevation of CA 125 in serum has generally reflected persistence of disease at second look surveillance procedures. CA 125 levels can, however, return to within normal limits and residual disease can be found at laparoscopy or laparotomy. CA 125 shows promise for distinguishing benign from malignant pelvic masses. Trials are currently underway to evaluate the potential of CA 125 in combination with other markers to facilitate earlier detection of occult ovarian cancer.