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Background and Objective: Urinary tract infection is the second common bacterial infection in children. Constipation as a risk factor in urinary tract infection was reported by several studies. This study was carried out to evaluate the relationship between rectal diameter and constipation with urinary tract infections in children. Methods: This case – control study was performed on 40, 2-12 years old hospitalized children with urinary tract infections as cases and 40 healthy children with the same age as controls in Boali Hospital in Sari, Iran. Rectal diameter was measured by ultrasonography. Demographic characteristics, rectal diameter, during of infectious and disease clinical signs were recorded for each subject. Results: Constipation was observed in 62.5% and 17.5% of cases and controls, respectively (P<0.05). Rectal diameter was 47.64 mm and 26.48 mm in case and control groups, respectively (P<0.05). Conclusion: Regarding to increasing of rectal diameter in patients with urinary tract infections in comparison with healthy subjects, ultrasonography can be a reliable method either to confirm or reject the presence of constipation in urinary tract infections.