Endoscopic ultrasonography (EUS) was performed in 110 patients with gastric cancer to examine the depth of cancer invasion. The normal gastric wall has a five-layer structure on EUS. In 19 lesions, changes on EUS were limited to the first and/or second layers, suggesting that invasion was limited within the mucosa. In 91 lesions, changes were evident in the third or deeper layers. Alterations in the EUS appearance could be divided into two types: type A, with destruction of the layered structure and type B with thickening of the layers. The amount of interstitial tissue in cancer was classified as medullary or intermediate in 90% of type A, and as scirrhous in 82% of type B tumor. The EUS diagnosis of invasion was 95% accurate for stage M cancers, 75% for SM, 64% for PM, 100% for SS, 79% for SE, and 100% for SI lesions. The overall accuracy was 81%. Accompanying fibrosis and metastatic perigastric nodes were the main reasons for overestimating the extent of disease.