In the Department of Paediatrics at Hillerød a valid diagnosis of urinary tract infection in children is defined to be at least 10.000 bacteria/ml. in two midstream specimens of urine or any bacterial growth in urine collected by a suprapubic aspiration or by bladder catheterisation. We decided to study if these criteria were fulfilled in our department, which takes care of urinary tract infections in children in the county of Frederiksborg with a catchment area of about 350.000 people. From 1994-1996 a total of 60 children younger than two years of age were treated for their first suspected urinary tract infection. The diagnosis was valid in 37% of the cases. Eighty-eight percent of the children received parenteral antibiotics for at least three days, and 86% were discharged with prophylactic antibiotics. All underwent urological ultrasonographic examination and 70% underwent an isotope micturition cystourethrography. Among patients with a valid diagnosis 68% were boys and 50% had urological abnormalities, whereas among patients without a valid diagnosis there were 33% boys (p <0.025) and 24% had urological abnormalities (p = 0.0734). A valid diagnosis of a first urinary tract infection was established in 0.15% of children younger than two years of age in the county of Frederiksborg. This is lower than previously reported. The problem of false positive urinary tract infections thus may be important in general.