The aim of this study was to determine whether circumcision affects significant bacteriuria in boys. During a 60-month prospective study, 100 boys with microbiologically confirmed symptomatic urinary tract infection (UTI) were evaluated. Twelve patients with abnormal ultrasonography findings were excluded from the study. Eighteen of the boys had not been circumcised due to parental choice. The remaining 70 boys with normal renal ultrasonography were randomly allocated into two groups. In the first group 35 boys ranging in age from 6 months to 10 years (mean 33.2+/-30.9 months) were observed for 6 months and urinary cultures were taken monthly. When they had a positive urine culture (with or without any symptoms), they received antibiotic treatment. After 6 months they were circumcised and then observed for another 6-month period. Group 2 comprised 35 boys aged from 3 months to 9 years (mean 29.1+/-36.7 months). They were circumcised immediately after the first UTI and were followed for 6 months. Urine samples were obtained by the bag technique in infants and by the mid-stream technique in older patients. In the uncircumcised group, the rate of significant bacteriuria per patient did not change in two 6-month follow-up periods (3.46+/-0.91 and 3.33+/-0.97 episodes). In group 1, the rate of positive urine cultures dropped from 3.57+/-1.11 to 0.14+/-0.35 episodes after circumcision (P<0.001). In the second group, the rate of significant bacteriuria was 0.17+/-0.38 episodes after circumcision. Among the uncircumcised patients, symptomatic UTI was observed in 6 cases (3 cases in the first period of group 1, 1 case in the first and 2 cases in the second period of the uncircumcised group), whereas after circumcision no patient had symptomatic UTI. The mean age at circumcision was 42.7+/-28.4 months. No complication due to circumcision occurred in any patient. UTI may also occur in boys after the 1st year of life. The present study indicated that circumcision in boys decreases the rate of positive urine cultures. Therefore circumcision could be considered as a part of UTI therapy.