Tumor markers are used for multiple purposes in clinical care, including screening asymptomatic subjects, differential diagnosis of symptomatic patients, treatment planning, prognosis during and immediately following treatment, and monitoring for recurrence. Generally, tumor markers have found most clinical utility in monitoring for recurrence of disease (1 ). Bast and coinvestigators discovered CA125 in 1979 using monoclonal antibody techniques (2 ), and subsequently demonstrated its utility in monitoring treatment and recurrence of ovarian cancer (3 ). CA125 is the most widely used ovarian tumor marker, and is currently approved in the United States for monitoring of disease to determine if second-look surgery is required. Tumor markers have not gained wide acceptance for early detection of disease with the one exception of PSA for prostate cancer in the U.S. The lack of acceptance is mainly because of the difficult hurdles a screening strategy must overcome, and few tumor markers have shown sufficient promise in overcoming these hurdles to put them to the test in a randomized controlled trial. Because of the low incidence of most cancers, sample sizes for prospective randomized screening trials are huge, so that sufficient numbers of disease specific events occur by the end of the trial. The significant costs entailed in clinical trials of this size imply that only very promising approaches to screening warrant prospective investigation. For ovarian cancer, three large trials are underway, two trials are planning to randomize 120,000 women followed for 7-8 yr (4 ,5 ), and an NCI trial will randomize 74,000 women followed for 16 yr (6 ).