In order to assess the utility of the tumor-associated antigen CA 72-4 in the diagnosis and monitoring of gastric cancer, this tumor marker was measured preoperatively in 718 patients. This group comprised 282 patients with malignant disease (115 with gastric cancer and 167 with other malignancies not involving the stomach) and 476 patients with benign surgical diseases. These results were compared with those for carcinoembryonic antigen (CEA) and the tumor-associated antigen CA 19-9. CA 72-4 was above the normal limit of 2.5 U/ml in 61% of the patients with gastric cancer, in 35% of the patients with other malignancies, and in 7% of the patients with benign diseases. CEA and CA 19-9 were elevated in 37% of the patients with gastric cancer ( > 3 ng/ml for CEA and > 37 U/ml for CA 19-9). CA 72-4 levels were above 10 U/ml in 26% of the gastric cancer patients, in 15% of patients with other malignancies, and in 0.4% of the patients with benign diseases. There was a good correlation between CA 72-4 level and tumor stage in gastric cancer. CA 72-4 serum levels were over 2.5 U/ml in 31%, 48%, 68% and 88% of patients with stage I, II, III and IV disease, respectively. CA 72-4 was found to be more sensitive than CEA and CA 19-9 in detecting recurrences of gastric cancer. In the postoperative-care period, carcinoma recurred in 29 patients.(ABSTRACT TRUNCATED AT 250 WORDS)