Retrospective analysis of forty one retroperitoneal masses was made with computed tomography. All cases wereconfirmed pathologically by operation or needle aspiration biopsy. The cases were; 14 abscesses, 2 leiomyosarcomas, 2 rhabdomyosarcomas, 2 liposarcomas, 1 undifferentiated sarcoma, 3 endodernal sinus tumors, 4 teratomas, 2 neurofibromas, 1 neurilemoma, 3 neuroblastomas, 4 milignant lymphomas, 2 malignant fibrous histiocytomas and 1 hemangiopericytoma. Tumors originated fromurinary tract and pancreas were not included. Findings favor tumor rather than inflammation were; large lobulated soft tissue mass wit eccentric or heterogeneous low density, thick cavity wall, disruption of fascial planes, regional lymphnode enlargement and distant metastasis.