A large, poorly demarcated, elevated lesion over the area ranging from the lower end of the esophagus to the lesser curvature of the cardiac region was examined in a 19 year old woman, and demonstrated an irregular and highly infiltrative proliferation of spindle cells over the full thickness of the gastric wall. Although the spindle cells grew mainly in the submucosa, they penetrated the muscularis propria, leaving intact muscle tissue that formed an irregular, island-like pattern, and reached the subserosa. The cells were identified as myofibroblasts by light microscopy, immunohistochemistry and electron microscopy. The present case is considered to be of an inflammatory pseudotumor due to extremely infiltrative growth of myofibroblasts. This lesion differed from inflammatory fibroid polyp (similarly consisting of proliferating myofibroblasts accompanied by inflammatory cells) in terms of age at onset, tumor size, pattern of cell proliferation and main inflammatory cell component. This report also discusses the relationship between this lesion and a very recently advocated new entity, inflammatory fibrosarcoma of the mesentery and retroperitoneum, which is a tumour closely simulating inflammatory pseudotumor.