The incidence and the prophylaxis of urinary tract infection following bladder catheterization have been studied in 142 patients admitted for voiding cystourethrography (VCU). 71 children received a prophylactic treatment of 2 mg/kg nitrofurantoin daily for 4 days, while 71 received no treatment following bladder catheterization. Bacteriologic examination of the urine 4-9 days after catheterization showed significant bacteriuria in 5 untreated children. The germs responsible were E. coli, Proteus, and Klebsiella. None of the treated patients presented with significant bacteriuria. It is concluded that the incidence of significant bacteriuria after a VCU is not inconsiderable, that this urinary tract infection is often asymptomatic, and that the prophylactic administration of a disinfectant is justified.