We report on a patient with a gastric leiomyosarcoma that had a giant cystic component and very high levels of ferritin in serum and cystic content. A 38-year-old man presented with an upper abdominal mass. Upper gastrointestinal series and gastroscopy revealed a submucosal tumor showing central retraction in the upper part of the stomach. Abdominal computed tomography as well as ultrasonography showed the tumor as a huge cyst with multiple septa. Selective arteriography demonstrated a tumor of the stomach. Histological examination suggested a highly malignant leiomyosarcoma, extending beyond the stomach, the serum ferritin level was very high: 307.8 ng/ml in serum and 22,800 ng/ml in the cystic content. This has, to our knowledge, not been reported before.