Determination of the resection margins during surgery for gastric malignancy is a subject of controversy because accurate detection of horizontal limits of tumor spread is difficult by current methods. In this study, we investigated the value of intraoperative-ultrasonography (IUS) in the detection of proximal and distal limits of horizontal tumor spread (HTS) in 19 gastric adenocarcinoma (Group I) and five gastric lymphoma (Group II) patients. After sonographic and clinical limits of HTS were marked, resections were carried out 2 cm away from the IUS limits, and biopsies from IUS limits and resection margins were obtained. Then, the gap between IUS and clinical limits were recorded and compared with results of pathology. In both groups, the distance from clinical to IUS limits ranged from 0 cm to 3 cm and IUS limits were never inside the clinical limits. In Group I, results of sonographic proximal limit detection were satisfactory in all patients excluding recurrent cases (n:2) and a patient with early gastric cancer. Sonographic accuracy of distal limit detection was 68%. In Group I, only one resection margin (distal) was found to be infiltrated with cancer. In Group II, the success rate of IUS-guided HTS detection was 40%. IUS seems to be a valuable method for determining the extent of HTS during operation for gastric adenocarcinomas, especially so for the most problematic proximal limits. In recurrent cancers and lymphomas, however, results of IUS may be frequently misleading. IUS can be advocated as a routine procedure to determine the tumor limits and thus the resection margins during surgery for primary gastric adenocarcinomas.