Schwannomas are tumors originating from any nerve that has a Schwann cell sheath. Gastrointestinal (GI) schwannomas represent only 3% of all GI mesenchymal tumors. The stomach is the most common site of GI schwannomas, and schwannomas account for 0.2% of all gastric neoplasms. This report presents two cases of gastric schwannomas showing increased [18F]fluorodeoxyglucose (FDG) uptake on positron emission tomography (PET; maximum standardized uptake value 7.10 and 6.05). Additional immunohistochemical staining of glucose transporter type 1 (GLUT1) and the autocrine motility factor (AMF) was conducted after the tumors were resected, to identify the mechanism that increased FDG uptake on PET. Immunohistochemical expression of AMF was positive in both cases, whereas GLUT1 was negative. Autocrine motility factor is also known as phosphoglucose isomerase. However, the mechanism by which FDG is accumulated in schwannoma cells is uncertain, and may be related to intracellular glycolytic activity.