An 18-year-old asian woman presented to our surgery department with abdominal pain, discomfort, and a palpable mass. About several months previously, this patient felt abdominal discomfort. However, this patient did not further evaluation. The patient had no other previous medical history. The patient had no personal or family history of similar illnesses. The initial blood pressure was 120/80 mmHg; heart rate, 86 beats/minute; respiratory rate, 14 breaths/minute; and body temperature, 36.9 °C at admission. Her bowel sounds were normoactive and regular. Physical examination revealed non-specific tenderness throughout the abdomen. However, a very large mass was palpated from the epigastric area to the pelvic area. The initial laboratory examination revealed normal levels of leukocytes and hemoglobin. Abdominal computed tomography revealed a large fatty mass with a complex soft tissue component. The tumor was approximately 30 cm × 20 cm × 10 cm in size and filled the entire abdominal cavity. The tumor pushed up the abdominal viscera from the pelvic cavity. The results led us to consider retroperitoneal neoplasm, including retroperitoneal liposarcoma.