The primary role that tumor markers for cancer of the colon and rectum have at this time is for postoperative surveillance of those patients resected for cure who are at risk for recurrence of the disease. Carcinoembryonic antigen (CEA) serum levels are followed by most surgeons prospectively after the potentially curative resection. This tumor marker has not been advocated as a screening test for these cancers; however, a preoperative CEA serum level is determined in those patients before the initial surgery for colon or rectal cancer. The serum level of CEA is mainly determined by tumor differentiation and stage of disease. If the CEA serum level begins to increase during the postoperative surveillance period, the recurrence of colon or rectal cancer must be suspected. Further investigations are then performed to identify the location and resectability of the recurrent disease. Monoclonal antibodies labeled with radioisotope are presently being used clinically to identify recurrence of colon and rectal cancer. Used in conjunction with elevated serum CEA levels (or other determinants of recurrent disease) these tumor markers can specifically identify site(s) of cancer recurrence. Theoretically, by attaching cancer-fighting agents (i.e., chemotherapeutic agents) to the monoclonal antibody, the site of tumor recurrence can be potentially treated, too. Hence, these "tumor-seeking missiles" may one day be used to treat cancer recurrence.