A total of 306 children with grade III to IV vesicoureteral reflux (international classification) and a history of documented urinary tract infection was randomized into medical (155 patients) or surgical (151 patients) treatment arms in the European portion of the International Reflux Study in Children. Children treated medically were maintained on prophylactic antibacterials as long as the reflux persisted, while those treated surgically were covered prophylactically until followup studies at 6 months postoperatively demonstrated the reflux to be corrected. Standard definitions for bacteriuria were used, and the distinction was made clinically among acute pyelonephritis, cystitis and asymptomatic bacteriuria, supported in many instances by additional laboratory testing. Urine was cultured after 3 months and whenever suspicious symptoms occurred. Urinary tract infections developed during the first 5-year followup period in 59 patients (38%) in the medical group and in 59 (39%) in the surgical group but the incidence of pyelonephritis was higher in the medical group (21%) than in the surgical group (10%) (p < 0.01). Pyelonephritis often followed catheterization or cystoscopy but asymptomatic bacteriuria was uncommon after these procedures in either group. Recurrent infections were related to age, sex and treatment center. They were common in boys and girls entering under 1 year of age but were less common in girls and rare in boys entering after 1 year of age. Recurrences were lowest among the Finnish children and highest in the German and Belgian children.