BACKGROUND: Ultrasonography has shown to be useful in the diagnosis of constipation. OBJECTIVE: The aim of this study was to compare the rectal diameter and rectal wall thickness in children with and without constipation. METHODS: Children with the diagnosis of constipation according to Rome III criteria were included in the study. The children underwent transabdominal sonography for the evaluation of rectal diameter and rectal wall thickness. Ultrasonography was performed with a full bladder. Children without constipation who underwent abdominal sonography were assigned to the control group. RESULTS: The rectal diameter was larger in children with constipation than in children without constipation (31.72±9.63 mm vs 19.85±4.37 mm; P=0.001). The rectal wall was thinner in children with constipation than in children without constipation (1.75±0.33 mm vs 1.90±0.22 mm; P=0.032). There was no significant difference between boys and girls with constipation in terms of rectal diameter (31.02±8.57 mm 32.77±11.35 mm; P=0.63). CONCLUSION: Transabdominal rectal diameter measurement may be useful in the diagnosis of constipation.