Our patients have demonstrated that serial carcinoembryonic antigen determinations contributed to the detection of recurrent tumor and that shortening the delay between carcinoembryonic antigen elevation and reoperation has resulted in an increase from 27 to 78 per cent in instances of resectable recurrent tumor encountered. If these results continue to be substantiated, the carcinoembryonic antigen assay has made a significant contribution in the control of this disease. Serial carcinoembryonic antigen assays should be performed every two months. All benign inflammatory conditions that cause carcinoembryonic antigen elevations must be searched for, and ruled out, before reoperation is decided upon. The physician must be cognizant not only of the significance of the assay but also of the limitations, and he must rely heavily on his clinical judgment.