Renal involvement during lymphoma can be extrinsic, i.e., renal compression or displacement due to lymph node masses, or intrinsic, i.e., parenchymal involvement secondary to blood or lymphatic spread, or primary, as initial neoplastic site. Primary renal lymphoma is very rare (3% of all renal lymphomas) for the absence of lymphatic tissue in the kidney. The disease might be due to parapyelic lymph nodes or to blood spreading from an unknown site. In our study we reviewed the CT findings of five cases of primary non-Hodgkin's renal lymphoma with surgical or histologic confirmation. Renal alterations due to lymphomatous involvement were classified according to macroscopic pathologic findings: type I (single nodular disease, 2 patients), type II (multinodular disease, 1 patient), and type III (infiltrating disease with retroperitoneal involvement, 2 patients). In the two patients with single nodular involvement (type I), CT showed a solid, hypodense and clear-cut nodule. In the only patient with multinodular disease (type II), renal tissue was replaced by multiple hypodense nodular masses, which were partially confluent. In the two infiltrating forms with retroperitoneal involvement (type III), renal structure was diffusely disorganized, with thickening of soft tissues and perirenal fasciae, peripyelic infiltration and, in one case, urinary tract obstruction. To conclude, CT always allowed the accurate assessment of the presence, site and size of renal lesions and of perirenal and urinary involvement. However, CT findings were completely aspecific, not allowing an unquestionable differential diagnosis with other conditions, e.g., hypernephroma, transitional cell carcinoma, metastatic lesions or chronic inflammations. Therefore, a biopsy specimen is necessary to make an unquestionable diagnosis.