Aim of the work: accuracy of multi-detector computed tomography (MDCT) in the diagnosis of different retroperitoneal masses, by comparing the diagnosis reached by MDCT with the pathology results of the studied retroperitoneal cases. Background: cases suspected of retroperitoneal masses by ultrasound, (MDCT) is the reference standard for them, it shows up the accurate diagnosis, location, extent of the tumor, along with the involvement of adjacent organs and vessels. It also has high accuracy in the differentiation of malignant from benign lesions, detection of lymph node involvement & metastasis. Therefore MDCT plays an important role in treatment planning and follow-up of different retroperitoneal masses. MDCT has many Advantages over other imaging modalities: less invasive, more widely available, lower dose of radiation, higher spatial & temporal resolution Methods: This study was carried out at Radiodiagnosis Department, Zagazig University Hospitals, and other radiological centers. The present study was carried on 42 patients with retroperitoneal masses. Those patients were examined by MDCT. Results: The MDCT images of 42 patients were analyzed for reaching the final diagnosis. The results of MDCT were compared with the histopathological results. Positive results were obtained in 37 lesions out of a total of 42 cases, with estimated CT accuracy of about 88.1% in the diagnosis of different retroperitoneal masses. Conclusion: MDCT should be used in the first place in the course of diagnostic and staging procedures of patients with different retroperitoneal tumors and tumor recurrence.