AbstractWe have conducted a prospective study of 441 patients, to investigate the utility of a new tumor marker CA 19–9 for the diagnosis and monitoring of patients with cancer of the gastrointestinal tract (93 patients with colorectal carcinoma, 57 with carcinoma of the stomach, 10 with esophageal carcinoma, 45 with malignancies outside the gastrointestinal tract, and 236 with benign general surgical disease). Results were compared to those obtained for carcinoembryonic antigen (CEA) in the diagnosis of carcinoma of the stomach and colon/rectum. CEA is more sensitive than CA 19–9 in all stages of carcinoma of the stomach and colon/rectum. During treatments of gastrointestinal carcinomas, CEA and CA 19–9 were determined at the same time in 66 and 165 patients with surgically treated carcinoma of the stomach and colorectal carcinoma, respectively. It was noted that CEA is more sensitive than CA 19–9 in detecting recurrence. However, CA 19–9 is more specific. The best results were obtained when both markers were used together.