 This multicenter, randomized, double-blind, placebo-controlled trial was designed to determine whether daily antimicrobial prophylaxis is superior to placebo in preventing recurrence of urinary tract infection UTI in children with vesicoureteral reflux VUR. Eligibility criteria are described elsewhere. Patients were randomly assigned to treatment for 2 years with daily antimicrobial prophylaxis trimethoprim-sulfamethoxazole or placebo. The study was designed to recruit 600 children approximately 300 in each treatment group. The protocol encouraged prompt evaluation of children with UTI symptoms and early therapy of culture-proven UTIs. It was expected that approximately 10 of children will have to discontinue study medication due to allergic reactions. Assuming a 20 placebo event rate and 10 non-compliance rate, the study has 83 power to detect an absolute 10 event rate in the antimicrobial prophylaxis group. If the placebo event rate is instead 25, power is 97 to detect an absolute 10 event rate in the treated group, even if non-compliance is as high as 15. The primary analysis is intention-to-treat with missing outcome data analyzed as UTI. 