Sixty-three calculi in 25 kidneys were detected in 17 patients on noncontrast CT. Forty-seven (75%) of the 63 calculi were also detected on the arterial phase scans. The size and highest attenuation of calculi (measured on noncontrast CT) detected on arterial phase scans were 4.3 +/- 3.0 mm and 556 +/- 353 Hounsfield units, and those detected only on noncontrast CT were 2.8 +/- 0.8 mm (P = 0.053) and 181 +/- 47 Hounsfield units (P<0.0001), respectively