The pursuit of the ideal tumor marker has generated many tests for use in the diagnosis and management of cancer, several of which are now widely available. Tumor markers have five potential uses in patient care: They can be used for screening, for diagnosis, for establishing prognosis, for monitoring treatment, and for detecting relapse. The value of a marker in a given setting depends on two marker-related characteristics--sensitivity and specificity. The value of a marker in a particular malignancy also depends on the effectiveness of therapy for the malignancy. Tumor markers have been used to screen for occult cancer but have proved to be valuable only in selected cancers. As diagnostic tools, tumor markers have limitations: Nearly all markers can be elevated in benign disorders, and most markers are not elevated in the early stages of malignancy. Extreme marker elevation often indicates a poor prognosis and in some malignancies can indicate the need for more aggressive treatment. Tumor markers have their greatest value when used to monitor therapy in patients with widespread cancer. Nearly all markers show some correlation with the clinical course of disease, with marker elevation in any stage declining to normal after a curative intervention. Recurrent disease can be accompanied by increased marker levels, but markers can detect an occult recurrence in only a few diseases, thereby facilitating a second attempt at cure. Although it seems unlikely that an ideal tumor marker will be identified for every malignancy, several workable markers are already available. Increasing our knowledge about the capabilities and limitations of existing markers will enable us to use them judiciously in the treatment of cancer.