Objective: Zonulin is a protein that affects the integrity of intercellular connections inthe intestines. It has been emphasized that autoimmune diseases as well as neurode velopmental disorders, for example autism spectrum disorder (ASD), may occur throughalterations in intestinal permeability and blood-brain barrier. We aimed to investigate thegastrointestinal permeability of individuals with ASD by determining serum zonulin levelsand their relationship to symptom severity. Methods: Twenty-five ASD patients and 19 controls were included. Serum zonulin lev els were measured by enzyme-linked immunosorbent assay kits. Clinical severity wasassessed by the Childhood Autism Rating Scale (CARS), and social skills of the controlgroup were evaluated by the Conners’ Parents and Teacher’s Rating Scales-Revised/LongForms (CPRS-CTRS). Results: Mean zonulin levels were significantly higher in the ASD group and positivelycorrelated with CARS scores. After regression analysis, serum zonulin levels predictedCARS total scores. We could not find any significant correlation between zonulin levelsand CPRS-CTRS sociability subscale scores in the control group. Conclusion: The positive correlation between serum zonulin levels and ASD severity mayrequire precaution for impaired intestinal permeability in clinical practice, especially forthe cases in which sociability is severely impaired. However, it is too early to state thatintestinal permeability has a role in the etiology of ASD. Further studies involving specificautism subgroups, and samples with certain dietary differences are needed.