The outcome of using prophylactic antibiotics in children considered at risk for a urinary tract infection (UTI) was documented in 66 children during the period of suppressive antibiotics and for a follow-up period of 3.7+/-2.2 years (range 0.92-9.83 years). A breakthrough UTI occurred in 13 girls but none of the boys during the initial course of prophylactic antibiotics. During the follow-up period, 33 girls and 5 boys had no recurrence of infection, while 25 girls and 3 boys had UTIs. Statistical analysis of the data using chi-square and risk estimate relating factors for infection to the occurrence of a UTI showed that during the period of initial prophylactic antibiotic there was significant risk of infection among children with voiding dysfunction and abnormal kidneys and during the follow-up there was increased risk of infection among those with voiding dysfunction and vesicoureteric reflux (VUR) of grade 3 or greater severity. Lesser grades of VUR and constipation did not significantly increase the risk of UTI. These observations should be useful in developing a study to define the risks and benefits of prophylactic antibiotic in "at-risk" children.