In view of possible injury to bladder function in association with the open antireflux procedure, the influence of endoscopic instillation of Deflux ® was evaluated in a prospective randomised study. Supported by grant IGA MZCR 8021-3.Between 2003 and 2006, 37 children with primary vesicoureteral reflux grade III-V were randomly assigned into two groups: 21children (15 girls, 6 boys) aged 7-39 months (mean 20.3) were operated on by the instillation of Deflux ® - Group A; 16 children (8 girls, 8 boys) aged 1-47 months (mean 15.1) treated conservatively by antibiotics – Group B. During follow-up of 4 to 47 months (mean 18.1), repeated ultrasound and videourodynamic investigations were performed. Antibiotic treatment was stopped after the disappearance of reflux. Clinical data taken from parents (voiding habits, incontinence, urinary stream, straining or pain during micturition), and urodynamic data – capacity, intravesical pressure and residuals were evaluated.In the Group A, one 3.5 year-old girl suffered from postoperative worsening of incontinence without urodynamic pathology, in a 4-year-old boy, detrusor hyperactivity without clinical symptoms was detected and one 3-year-old girl suffered from pyelonephritis due to a temporary obstructive megaureter after instillation. In Group B, interrupted voiding in one 4.3-year-old girl was reported, and in another 4-year-old boy with persistent night-time incontinence bladder hyperactivity was detected. Four children suffered from urinary tract infection (febrile in three).Endoscopic instillation of Deflux ® was not associated with any damage to bladder function in comparison with the conservatively treated patients during midterm follow-up.