To clarify the significance of immunoreactive carcinoembryonic antigen in gastric juice (gastric CEA) from gastric cancer patients, we studied the gastric CEA in comparison with cancer progress, histologic types of tumors, staining for CEA and extent of intestinal metaplasia. The gastric CEA levels from patients with other gastric diseases, i.e., gastric ulcer, duodenal ulcer and chronic gastritis were also investigated. The mean gastric CEA level from gastric cancer patients was significantly higher than from gastric or duodenal ulcer patients, but there was no significant difference between levels in patients with gastric cancer and in those with chronic gastritis. The gastric CEA levels from cancer patients increased with progression of the cancer. The differentiated carcinomas showed significantly higher gastric CEA levels than the poorly differentiated carcinomas. Positive CEA tumors showed significantly higher gastric CEA levels than did the negative CEA tumors. The tumors with diffuse intestinal metaplasia revealed high gastric CEA levels. Determination of gastric CEA levels is considered to be useful for screening of gastric cancer.