The CT examinations and medical records of 33 patients with primary retroperitoneal malignancies were reviewed. Computed tomography findings were then compared with those from scans performed on 122 patients with non-Hodgkin lymphoma who presented during the same time interval. Primary retroperitoneal neoplasms had three distinct CT appearances. Twenty-one patients (64%) had large soft tissue masses. Seven patients (21%) had masses with fatty density components, and five patients (15%) had tumors that were primarily of water attenuation. With the exception of those liposarcomas that contained recognizable fat, CT could not distinguish among the different cell types. Seventeen patients with non-Hodgkin lymphoma had large dominant retroperitoneal soft tissue masses that resembled primary retroperitoneal malignancies. In most cases, however, CT was able to differentiate these tumor masses from primary retroperitoneal tumors. Although 19 of 20 sarcomas of soft tissue attenuation were heterogeneous, only six of the 17 lymphomas presenting as dominant masses had such an appearance. Computed tomography is extremely helpful in initially evaluating patients with primary retroperitoneal tumors and in assisting the surgeon in planning his or her approach by accurately defining tumor extent.