Aim: Alterations in gut-brain axis of patients with attention deficit hyperactivity disorder (ADHD) have been indicated in recent studies. The aim of this study was to determine whether serum level of zonulin, considered to be a regulator of permeability in both gut-blood and blood-brain barriers, was associated with ADHD symptoms, and to evaluate the relationship between zonulin and social cognition in children with ADHD.
 Material and Methods: Serum level of zonulin was analyzed by enzyme-linked immunosorbent assay (ELISA) in 40 treatment-naive children with ADHD, and age and gender matched 40 healthy children as control group. DuPaul ADHD Rating Scale was used for ADHD symptoms and reading the mind in the eyes test (RMET) was administered by the clinician to examine the social cognitive abilities.
 Results: We found that serum zonulin levels were significantly higher in ADHD group compared to control group (p=0.010). Also, children with ADHD have significantly lower RMET scores (p=0.007). Furthermore, we found statistically significant positive correlations between serum zonulin levels and ADHD symptoms (p