A prospective blind study comparing the findings of ultrasonography, intravenous pyelography, and voiding cystourethrography was conducted on 81 patients to examine the place of ultrasonography in the initial radiologic evaluation of children with urinary tract infection. The patients'' mean age was 4.8 years; 15 were male. Forty-eight were inpatients (mean age, 3.2 years) and 33 were outpatients (mean age, 7.2 years). In 29 patients (35.8%) abnormality of the urinary system was detected by one or more of the three imaging procedures; 21 were inpatients and eight were outpatients. The most frequent finding was vesicoureteral reflux, occurring in 62.1% of the pathologic cases. The findings at ultrasonography correlated well with those of intravenous pyelography in 73 of the 81 studies (90.1%), but they failed to demonstrate double collecting systems and several of the minor changes. However, ultrasonography in combination with cystourethrography identified all patients who had abnormal urinary systems, except for two children with negligible findings. Moreover, ultrasonography and cystourethrography together identified all 11 patients, nine of them inpatients, in whom surgical treatment was indicated. It is concluded that ultrasonography can successfully replace intravenous pyelography as a screening imaging procedure for the urinary system, but because of the superiority of intravenous pyelography in the detection of some types of lesions, intravenous pyelography will be required whenever ultrasonography or cystourethrography results are abnormal. Accordingly, and in view of the differences in the frequency and severity of pathologic findings between outpatients and hospitalized patients, the following protocol is suggested for the radiologic evaluation of children with urinary tract infection: For outpatients, cystourethrography can be performed 4 to 6 weeks after cessation of antibiotic therapy. If the study is normal, ultrasonography can be done; if this is also normal, no further radiologic workup is needed. Only when cystourethrography or ultrasonography findings are abnormal is intravenous pyelography also indicated. For hospitalized patients, especially young children, ultrasonography can be used as the early screening procedure, within two to four days after the diagnosis of urinary tract infection. If the results are normal, cystourethrography can follow after 4 to 6 weeks; if abnormal, cystourethrography can be performed after ten to 14 days. Here, too, intravenous pyelography is needed only when ultrasonography and/or cystourethrography results are abnormal.