Operative ultrasonography was performed during operations to improve the accuracy of diagnosing the extent of the spread of carcinoma of the stomach in the following areas: tumor invasion in the gastric wall (31 patients), para-aortic lymph node metastasis (30 patients) and hepatic metastasis (30 patients). The over-all diagnostic accuracy rates of operative ultrasonography were 81 per cent for determining the depth of tumor invasion, 80 per cent for carcinoma of the mucosa, 70 per cent for carcinoma of the submucosa, 83 per cent for carcinoma of the proper muscle, 80 per cent for carcinoma of the subserosa and 100 per cent for carcinoma of the extraserosa. The over-all accuracy of operative ultrasonography in diagnosing para-aortic lymph node metastasis was 93 per cent. In comparison with the results of preoperative studies, the accuracy (sensitivity) of operative ultrasonography was significantly superior in diagnosing metastasis to the lymph nodes. In three of 30 operations, preoperatively unrecognized and nonpalpable (occult) hepatic metastases were identified by operative ultrasonography. Operative ultrasonography provides more accurate information regarding the spread of carcinoma of the stomach, and thus may be helpful in deciding upon the type of surgical procedure or in avoiding unnecessary tissue dissection.