Routine monitoring of carcinoembryonic antigen (CEA) levels is standard in patients with resected colorectal cancer (CRC). The incidence of false-positives and the upper limits of false-positive elevations have not been previously well characterized. The computerized medical records at Memorial Sloan-Kettering were searched electronically, and then manually confirmed to identify patients who underwent an R0 resection of locoregional CRC between January 2003 and December 2012 and who had an increase in CEA level above the normal range after a normal perioperative CEA level. This search identified 728 patients who met all inclusion criteria. Of these, 358 (49%) were determined to have had a false-positive elevation of CEA level, 335 (46%) had a true-positive elevation of CEA level indicative of recurrent CRC, and 35 (5%) had a true-positive elevation of CEA level indicative of development of a new, non-CRC malignancy. Of those with false elevations, 111 had a single isolated CEA level elevation (median highest CEA level of 5.5 ng/mL; range, 5.1–45.2 ng/mL) with no further abnormal measurements, whereas 247 had CEA elevations on 2 or more readings, with a median highest CEA level of 6.7 ng/mL (range,5.2–34.9 ng/mL). Of these 247 patients with confirmed false-positive CEA level elevations, only 5 (2%) had measurements greater than 15 ng/mL, and no confirmed CEA elevation greater than 35 ng/mL was a false-positive. False-positive CEA test results in the range of 5 to 15 ng/mL are common. Confirmation of CEA elevation in this range prior to initiating imaging studies may be appropriate. False-positive CEA test results greater than 15 ng/mL are rare, and all confirmed CEAs greater than 35 ng/mL were associated with cancer recurrence.