The underlying mechanisms of urinary-tract infections (UTI) in renal transplant recipients are still not fully understood. In otherwise healthy children, bladder dysfunction increases the susceptibility to UTI. The aim of this study was to evaluate lower-urinary-tract function in children and adolescents after renal transplantation. Sixty-eight recipients of renal transplants, 5-20 years of age and 1-15 years after transplantation, were evaluated for their bladder function with a questionnaire, uroflowmetry and bladder ultrasound, and for renal function (glomerular filtration rate) by measuring clearance of inulin or iohexol. Forty-nine patients (72%) had some type of abnormality of bladder function. Abnormal bladder capacity was found in 26%, abnormal urinary flow in 50% and residual urine in 32% of the patients. There was no significant difference in bladder or renal function in children with urinary-tract malformations compared with those with normal urinary tract. Furthermore, there was no significant difference in renal function in patients with bladder dysfunction compared with those without. The incidence of bladder dysfunction is high in children and adolescents after renal transplantation, but the clinical significance of this finding and whether there is a correlation between bladder dysfunction and UTI in these patients need to be clarified further.