Ultrasonography of the kidney may replace the intravenous pyelogram as the study of choice in identifying treatable abnormalities in children with urinary tract infection. In a series of 64 consecutive children with urinary tract infection in whom an intravenous pyelogram, renal ultrasound, and voiding cystogram were performed, only one treatable abnormality (calyceal dilation) was identified by intravenous pyelogram, and it was not detected by ultrasound. Eleven children showed vesicoureteral reflux on the cystogram. In an additional 43 children in whom intravenous pyelogram was done only if the ultrasound or cystogram were abnormal, there were five abnormal cystograms. Four treatable abnormalities were identified by ultrasound, and there were confirmed by the intravenous pyelogram. Ultrasound should replace the intravenous pyelogram in children with a normal cystogram because of its accuracy, safety, and high patient acceptance. We have also documented a significant volume increase with acute infection in one or both kidney(s) of those children having upper urinary tract infection. Fifteen of 18 children with upper urinary tract infection had volume increases of 30% or more in at least one kidney; whereas only four of 21 children with lower urinary tract infection had increases of greater than 30% (P less than .005). Ultrasound volume measurements provide a new, noninvasive method for identifying the probable site of urinary tract infection.