A 36-year-old man presents with weight loss and a large palpable tumor in the upper abdomen. Endoscopy reveals an intact gastric mucosa without signs of carcinoma. Multiple biopsies show normal gastric mucosa. A UGI study shows a mass in the stomach. At surgery, a 3-kg mass is removed. It is necessary to remove the left side of the transverse colon. What is the most likely diagnosis?

GISTs were previously called leiomyosarcomas. They are rare (4% of all gastrointestinal tumors). They can cause confusion because the overlying mucosa may remain intact. They grow slowly, invade locally, and are not responsive to radiation or chemotherapy. Eosinophilic gastroenteritis is an infiltrative lesion that usually involves the gastric antrum. It is of unknown origin and differs from Menetrier's disease, where the mucosal folds of the proximal stomach are initially involved.