Materials and Methods: In this case-control study, 4-12-year-old children who were diagnosed with functional constipation were included in the case group and children with no functional constipation were included in the control group. Diagnosis of functional constipation was performed according to ROME III criteria. All participants were interviewed for ADHD using the Persian version of Conners" Parent Rating Scale (short form). Statistical analysis was performed by Chi-square test for investigating the relationship between functional constipation and ADHD.

Results: A total of 100 children with functional constipation and 147 children without constipation were selected. Among the functional constipation group, 27 (27%) had a high score for ADHD and among the group without functional constipation, 16 (10.0%) had a high score for ADHD. The odds ratio was 3.028 (95% confidence interval = 1.532-5.986; P value = 0.001). There was no statistically significant relationship between constipation and ADHD considering the age and gender.

Conclusions: Functional constipation has a relationship with ADHD. The chance of having ADHD is 3 times higher in children with functional constipation. Further studies, particularly cohort studies, are recommended to assess the incidence of ADHD symptoms in infants with functional constipation in the future. Further studies on the possible pathophysiology of the digestive system and bacterial intestinal flora, especially in early childhood and infancy, are suggested for investigating the development of behavioral disorders and ADHD.