Background: Vesicoureteral Reflux (VUR) is the most common urologic anomaly in children. In spite of prescribing prophylactic antibiotics in children with VUR, in some cases breakthrough infection (BTI) occurs while receiving prophylactic antibiotics which may cause pyelonephritis that can ultimately lead in renal scarring.

Materials and Methods: In this discreptive-analytic study, 288 children with primary vesicoureteral reflux under Cotrimoxazole(2mg/kg) treatment have been under supervision for duration of at least one year. Based on the collected data, predictive effect of some factors like gender, mean age of VUR diagnosis, VUR grade, unilateral versus bilateral and presence of abnormal renal scar on the breakthrough infection under prophylactic treatment, were examined.

Results: Breakthrough infection while receiving prophylactic antibiotics was observed among 111(38.54%) patients consisting of 88 girls (79.28%) and 23 boys (20.72%). The moderate to high VUR grades (grade III-V), presence of renal scar and female gender were found to be the important risk factors for BTI. However, no statistically significant relationship between mean age of VUR diagnosis and unilateral versus bilateral VUR with BTI was found.

Conclusion: Results of the current study will help improving parents' and physicians' awareness of the risk factors associated with BTI which may potentially lead to renal damage.