Background: Impairment in the function of the lower urinary tract can be the cause of recurrent urinary tract infections (UTI) and vesico-ureteral reflux (VUR) in children. The purpose of our research was to evaluate the frequency of occurrence of bladder instability in children with UTI.Methods: The research involved 133 children (11 boys, 122 girls), ranging in age from seven months to 14 years. Group A consisted of 78 children with a history of recurrent UTI, while Group B included 55 children with recurrent UTI and VUR. Urodynamic tests (cystometry) were performed on all the children.Results: Abnormal functioning of the lower urinary tract was found in 98 children (73.1%) from Group A and 41 children (78.8%) from Group B. The most common dysfunction was detrusor-sphincter dyssynergia (DSD), which was found in 54% of all subjects, 46.2% of patients in Group A and 60% of patients in Group B (p<0.05). Unstable bladder was found in 42 (33%) children with no significant difference between the two groups. In 17 children (12.6%) DSD was accompanied by bladder instability. In both groups about 20% of the children did not present with symptoms indicative of urination dysfunction, where as 80% reported various symptoms, of which the most common were constipation and urinary urgency. In half of the children from Group A and one-fourth of the children from Group B there were several co-occurring symptoms: frequency, urgency, intermittent voiding, incontinence, dribbling and retention, and constipation.Conclusions: The most common disturbance of lower urinary tract function in these children with recurrent UTI was DSD, which occurred more often in children with VUR.