A large exogastric leiomyoblastoma in a 48-year-old male revealed by asymptomatic upper abdominal mass is reported. Abdominal ultrasound, computerized tomography scan and magnetic resonance showed a 20 cm cystic lesion apart from liver and pancreas of undetermined origin. During hospitalization, massive intraperitoneal bleeding due to rupture of the mass was observed. An emergency laparotomy was carried out, and excision of a large, ruptured, cystic mass involving the greater gastric curvature was performed. Microscopy revealed a gastric leiomyoblastoma. Even if infrequent, massive intraperitoneal bleeding is a very serious complication of gastric leiomyoblastoma. Considering the difficulty of an accurate preoperative diagnosis and the risk of intraperitoneal rupture, the authors suggest that similar abdominal masses should be managed by quick diagnostic investigations and early surgical procedures.