A 75-year-old woman presented with an 8-month history of progressive abdominal distension and associated weight loss. Computed tomography (CT) showed a very large, partially enhancing, cystic/solid mass within the abdomen that extended from below the left diaphragm to the pelvis. A large (10 cm), partially enhancing lesion was also seen in segments 6 and 7 of the right lobe of the liver, consistent with metastatic disease. The patient underwent laparotomy and resection of a 5.7-kg abdominal tumor measuring 30 x 27 x 20 cm. Histology confirmed a gastrointestinal stromal tumor (GIST) of stomach origin. A whole-body F-18 fluorodeoxyglucose (FDG) positron emission tomography (PET) scan performed 2 months postoperatively demonstrated multiple glucose-avid lesions in the liver as well as residual disease within the abdomen and pelvis. The patient was referred to medical oncology. The authors give a brief overview of this rare mesenchymal tumor of the gastrointestinal tract and the potential role of PET.