A 66-year-old woman presented with left chest pain. Left pleural effusion was seen on a chest X-ray film and a large mass disclosed by chest computed tomography. However, the patient refused to undergo a recommended operation. Six months later, she was admitted without any symptoms. A huge (18 cm diameter) mass was detected by magnetic resonance imaging (MRI), and consisted of heterogeneous solid and cystic components. Angiography and endoscopic sonography disclosed a suspected abdominal tumor, which was resected by thoracolaparotomy. Gastric stromal tumor was diagnosed on the basis of histological findings. Chest pain and pleural effusion are rare as initial clinical symptoms of such tumors.