A 40-year-old man complaining of abdominal distention was referred to our hospital. Computed tomography of the abdomen demonstrated a very large abdominal mass with fat and calcification. The size of the mass rapidly increased from 30cm to 40cm over two weeks. The tumor was removed and diagnosed by pathological examination to be a retroperitoneal mature cystic teratoma that contained a 40-cm long, mature intestinal tract-like cyst, together with bone marrow and fat. The rapid growth of the tumor may have been caused by an increased secretion in the cyst.