PURPOSE: To assess the incidence and causes of pseudohydronephrosis in the abdominal two-phase (arterial and portal) spiral CT. MATERIALS AND METHODS: This study included 115 renal pelvises of 62 patients (hepatocellular carcinoma in 36, gastric carcinoma in 10 and others in 16 patients) whose entire renal pelvises were imaged in both phases of abdominal spiral CT. Arterial and portal phase scans were obtained 30 and 65-70 seconds after the injection of 100 mL of non-ionic contrast media (370mg I/mL) at a rate of 2.5 mL/sec, respectively. The renal pelvis was considered to be dilated when the shortest diameter of renal pelvis exceeded 1cm as seen on CT. In each patient, a plain film was obtained 5-10 minutes after the injection of contrast media for CT to confirm the hydronephrosis. RESULTS: Seven renal pelvises were dilated on spiral CT. All of them were pseudohydronephrosis with prominent renal pelvis and two of them were extrarenal pelvises. In 115 renal pelvises, there was one case of true hydronephrosis due to a ureteral stone not suspected clinically. CONCLUSION: A 6% incidence of pseudohydronephrosis due to prominent renal pelvis should be considered in interpreting the abdominal two-phase spiral CT.