Urinary tract infections are the most common type of infection of bacterial origin in the paediatric population. The main aetiological factor for urinary tract infections are Gram-negative bacteria colonising the gastrointestinal tract, subpreputial area or vaginal vestibule. The most common cause of the infection is Escherichia coli. Quite frequently a urinary tract infection may be the first symptom of anatomical or functional abnormalities in the urinary tract; therefore, special attention should be paid to urinary tract infection incidents in the youngest age group. Cases of children aged 1–12 months during a first-time urinary tract infection episode hospitalised at the Department of Paediatrics, Paediatric Nephrology and Allergology of the Military Institute of Medicine in 2008–2014 were assessed retrospectively. The method of retrospective analysis of medical records was used. A group of 217 children was assessed, with girls slightly outnumbering boys. The most commonly observed symptom was fever, which was often accompanied by symptoms indicating a respiratory tract infection. This fact is worth emphasising since in such cases the presence of a urinary tract infection may be overlooked. Elevated inflammation parameters were observed in the majority of patients. The most common cause of the infection was Escherichia coli. A normal ultrasound image of the urinary tract was reported in only 17% of children. In 60% of patients who had a voiding cystourethrography no abnormalities were observed, in 30% vesicoureteral reflux was found and in 7% of such patients posterior urethral valves were the primary abnormality. Clinical symptoms and inflammation parameter values did not differentiate children with an isolated infection, vesicoureteral reflux and other urinary tract defects. In patients with urinary tract defects other than vesicoureteral reflux a different cause of the infection than Escherichia coli was significantly more common – strains of Gram-positive bacteria were more frequently found.