One of the major indications of tumor markers is the detection of occult disease. Less than 20 % of tumor markers elevations are associated with clinical or radiological findings. Such elevations have led medical community to doubt about the interest of markers follow-up, such as CA 15.3 in breast cancer. Now, positron emission tomography with (18)fluoro-desoxyglucose (18FDG-PET), using metabolic properties of malignant cells, is able to visualize tumor recurrences at an early stage of development, before any occurrence morphologic changes depicted by radiological examinations. Because of its cost and its limited accessibility, this functional technique should only be prescribed following a large set of informative indications, of which tumor markers belong. Early positive, non invasive and cost effectiveness, tumor markers become a precious guide in the prescription of 18FDG-PET in oncology. This article reviews the results of a set of recent studies in colorectal, breast and ovarian cancer.