CT findings of retroperitoneal neoplasma in 50 cases (51 tumors) were reviewed. There were 28 (55%) malignant tumors and 23 (45%) benign ones. MFH and liposarcoma were the most common malignant tumors and neural origin tumors were the most common benign ones. Differentiation is difficult on the basis of CT features alone. Benign tumors were usually smooth and well defined, and malignant ones ill-defined, irregular in shape, heterogenous in density with massive necrosis. The characteristic CT appearance of liposarcoma is the CT attenuation value by fat density. Neural origin tumors are usually located near the spine. They may have thick wall cystic appearance or are dumbbell shaped, showing expansion or extrinsic pressure to the adjacent bone structures. MFH, hemangiopericytoma and other malignant tumors may have marked enhancement after contrast administration. Non-resectability is shown as: 1. big vessels encased by tumor over 90 degrees, 2. adjacent organs or structures invaded by tumor, 3. multiple masses, and 4. huge tumor invading into the pelvis. Local recurrence is common after surgery. Follow-up CT scans every 6 months in a 2 year period is suggested for early detection of recurrence.