We have measured plasma levels of carcinoembryonic antigen (CEA) in 259 patients with a variety of cancers, 266 patients with noncancerous disease, and 176 healthy subjects. Sixty-six of 73 patients with malignant tumors of the gastrointestinal tract had elevated (> 2.5 ng/ml) CEA levels. There was no correlation between CEA level and extent of disease. Inconsistencies were noted between clinical status and CEA levels during follow-up of seven of 26 patients 3 to 96 months postoperatively. Elevated levels of CEA were detected in 117 of 186 patients with nonentodermally derived tumors, 83 of 183 patients with noncancerous disease, and 19 of 176 healthy individuals. The lack of specificity and sensitivity of the CEA test limit its usefulness as a screening procedure. Our data also suggest that it may have limited value in detecting recurrence.