The purpose of this study is to examine the applicability of contrast enhanced voiding ultrasonography in diagnosing vesicoureteral reflux (VUR) in pediatric patients compared to direct isotope cystography. The aim of the study is to find a modality allowing the accurate diagnosis of VUR and the classification of the degree of VUR with the least possible harm from the examinations. The investigators recruit 100 under 6 years old patients who have had at least 2 culture positive urinary tract infections. Exluding criteria are abnormal bladder function and posterior uretral valves. The investigators perform direct isotope cystography and contrast enhanced voiding urosonography during the same day to diagnose and grade VUR. The investigators use standardized volume and speed of bladder filling. The examinations are done by a radiologist and an isotope specialist and they are blinded to the result of the other examination.