We report an unusual case of a 76 year old woman admitted to our hospital for investigation of anemia and palpable epigastric mass. Intraoperatively a huge (19 cm), well defined tumor was found adhering to the stomach wall, protruding into the greater omentum and compressing the transverse colon. A tumorectomy was performed and the greater omentum was removed due to its close relation. Pathology revealed a high risk Gastrointestinal Stromal tumor of the gastric serosa. Histologically the tumor was of mixed type (spindle and epitheliod cells) with hemangiopericytomatoid pattern peripherically, variably myxoid stroma, central necrosis and cytologic pleomorphism. On immunohistochemical examination there was a consistent positivity for c-kit (CD-117) and CD-34, but without myogenic or neural differentiation. We consider this case unusual because of its huge size, its gastric serosal location and its extremely rare histological pattern.