The obliteration of a fat plane between the gastric carcinoma and the pancreas is a major criterion on CT scan for pancreatic invasion of gastric carcinoma. However, this sign is not always a reliable indicatior of invasion. as the patients with gastric carcinoma are often emaciated. producing a false positive CT findings. The purpose of our study is to improve the diagnostic accuracy of pancreatic invasion of gastric carcinoma in cases which the fat plane between the gastric carcinoma and the pancreas is obliterated in conventional CT scan. The authors performed lateral decubitus as well as supine CT scans and upper gastrointestinal series(UGIS) in 49 pathologically proven cases in which the fat plane was obliterated between the gastric carcinoma and the pancreas on conventional supine CT scan. Pancreatic invasion was suggested when the fat plane was obliterated persistently in the lateral decubitus view as well as the supine CT images and the involved gastric wall and adjace t pancreas maintained constant approximation despite postural change (CT+) and when the gastric tumor moved downward on the erect view of the UGIS no more than 1.5 time the height of the first lumbar vertevral body (UGIS+) Among 49 cases in which the fat plane between the gastric carcinoma and the pancreas was obliterated on supine CT scan. pancreatic invasion was confirmed pathologically in 11 cases (22.4%) Eight of 11 cases proven as pancreatic invasion were correctly diagnosed as pancreatic invasion by this comined analysis (CT+/UGIS+72.7%) Twenty seven of 38 cases proven as no pancreatic invasion were correctly diagnosed as no pancreatic invasion were correctly diagnosed as no pancreatic invasion (CT -/UGIS. 71.1%) Twelve cases showed CT+/UGIS-or CT-/UGIS +, so it was inconclusive whether there was invasion or not. The overall diagnostic accuracy was 71.4% In conclusion. combined analysis with supine and lateral decubitus CT and UGIS is useful improving diagnostic accuracy for pa ccreatic invasion by gastric carcinoma in patients of which the fat plane between the gastric carcinoma and the pancreas is obliterated on conventional supine CT.