An attempt is made to define the usefulness and limitations of carcinoembyonic antigen (CEA) radioimmunoassay for evaluation of diagnosis, tumor resection, and detection of tumor relapse in 108 patients with colorectal carcinoma. Preoperative CEA levels were correlated with pathologic stage and tumor localizations. Increasing levels of CEA were found with the advanced stage of the disease (stage C and D lesions). Our results indicate that 1) an incomplete drop in circulating CEA level one month after surgery is a bad prognostic sign and 2) relapses of colonic and rectal carcinoma can be detected by increased CEA levels months before the appearance of any clinical evidence