Abstract： Objective To investigate the structural and functional changes of internal anal sphincter (IAS) in children with functional constipation (FC),and to evaluate the association between the thickness of IAS and the severity of clinical symptoms.Methods A total of 35 children with FC (canstipation group,17 with incontinence) between June 2008 and December 2008 at the Shengjing Hospital of China Medical University were evaluated using anal manometry and endosonography.These patients were compared to 23 hospitalized children who were excluded for digestive and endocrinal diseases (control group).A validated symptom score (SS) was used to assess the severity of symptoms.The sum of SS ranged between 0 and 65.Results Anorectal manometry showed reflex relaxation of IAS in response to distension of rectal balloon in all patients.Rectal perceptional threshold in FC group was significantly higher than that in the controls [(42.4±19.5) ml vs.(29.1±15.6) ml,P＜0.05].The lowest volume for inducing reflex relaxation of IAS was significantly higher than that in the controls [(55.6±31.6) ml vs.(30.5±13.8) ml,P＜0.05].The thickening of IAS was noted in all the patients [(3.8±1.7) mm vs.(2.5±1.0) mm,P＜0.05].However,there was no significant difference between FC and control in median resting anal sphincter pressure[ (170.8±62.3) mm Hg vs.( 161.3±51.1 ) mm Hg,P＞0.05].The median symptom score was 9.3 ±4.3 in the FC group.The thickness of IAS correlated significantly with total symptom severity score (r=0.407,P＜0.05).There was no correlation between thickness of IAS and age,sex,or duration of disease(P＞0.05).Conclusions Structural and functional changes of internal anal sphincter exist in children with functional constipation.The thickness of internal anal sphincter correlates significantly with symptom severity.