Purpose:To assess the utility of multiphasic spiral CT for characterizing renal masses. Materials and Methods:The study included 36 patients(53 lesions) referred for the evaluation of renal masses suspected on the basis of the results of sonography or radiography. Spiral CT of the kidneys was performed prior to and following power injection of intravenous contrast material (Optiray-320). Postcontrast imaging data were obtained and analyzed during early and late corticomedullary(20-30-second delay), nephrographic(60-70-second delay), and excretory (5-minute delay) phases. During each phase, the ability to detect renal masses was evaluated, and the ability to diagnose these masses on routine and multiphasic CT was assessed. Routine precontrast and excretory phase CT studies were performed and the usefulness of each phase for the diagnosis of renal cell carcinoma(RCC) and transitional cell carcinoma(TCC) was evaluated. Results:The rates for the detection of renal masses as follows:94.3% on precontrast scan, 93.8% during the early corticomedulolary [hase(ECMP), 98.1% during the late corticomedullary phase(LCMP), 100% during the nephrographic phase(NP), and 98.1% during the excretory phase(EP). During both routine and multiphasic CT, diagnostic accuracy was 96.2%, thought for differential diagnosis, multiphasic CT was more helpful than routine CT in 4/16 cases of RCC and 2/8 cases of TCC. The highest for lesion characterization, during the LCMP in RCC, and the LCMP and NP in TCC;for evaluation of tumor margin during the EP in both RCC and TCC;for delineation of the renal artery, during the LCMP in both RCC and TCC;for delineating the renal vein, during the NP in RCC, and the LCMP in TCC. Conclusion:For the detection and correct diagnosis of lesions, multiphasic CT was superior to routine CT, but for the characterization of RCC and TCC, the former was helpful. The most useful phase can differ according to the kind of renal mass, and so for characterization of the mass, the most appropriate phase must be selected.