The authors reviewed the CT findings of 31 cased of primary retroperitoneal tumors confi-rmed by operation (25 cases) or needle biopsy(6 cases) at the Chonnam University hospital from Jun 1983 to August 1988 the size shape location margin of mass effect on the surrounding structures presence or absence of metastasis and the composition of the tumor were analyzed to find the differential points between benign and malignant tumors and charac-teristic findings of each tumors. The results were as follows; 1. The benign tumor showed smaller size than malignant one and well circumscribed round or ovoid lesions and no surrounding invasion or distant metastasis. 2. Some characteristic findings in teratoma liposarcoma and malignant fibrous histiocytoma aided in differential diagnosis; 1) Teratoma showed mixture of fatty low density and high density and characteristic fluid level 2) Lipogenic liposarcoma showed fatty low density slghtly higher than that of normal retroperitoneal fat. 3) Malignant fibrous histiocytoma showed well marginated mass with an attenuation coe-fficient equal or slightly lower than that of mormal muscle and arised mear kidney or adrenal gland and so compressed or invaded kidney.