A 33-year-old man visited the hospital with marked abdominal distention and vomiting. His medical history included epilepsy and mild mental retardation. He had no previous surgery or abdominal trauma history. Physical examination revealed a nontender, huge tumor palpable in the gross abdomen. Laboratory workup disclosed no abnormalities. An abdominal computed tomographic scan also showed a giant solid lesion occupying the entire abdominal cavity (Figure 1A). Angiography showed that the tumor was supplied by the bilateral internal iliac arteries, lumbar arteries, and branches of the superior and inferior mesenteric arteries (Figure 1B). Excisional biopsy under local anesthesia was carried out to confirm proliferation of spindle and epithelioid cells. Although the definitive diagnosis was uncertain at that time, exploratory laparotomy was carried out. He underwent en bloc tumor extirpation with resection of the sigmoid colon and distal ileum. The resected tumor was approximately 35 × 25 × 10 cm and weighed 4400 g. Pathological investigation revealed dense proliferation of spindle-shaped cells arranged in fascicles (Figure 2A). The immune profile of the tumor subsequently yielded positive expression of S-100 protein (Figure 2B), whereas there was negative staining of desmin.