To evaluate the relationship between the severity of autism, severity of gastrointestinal symptoms and serum zonulin levels as a marker of increased intestinal permeability in children. Serum zonulin levels were determined in 56 children with ASDs and 55 healthy children. The severity of gastrointestinal symptoms and ASD symptoms was assessed with the Gastrointestinal Symptom Rating Scale (GSRS) and Childhood Autism Rating Scale (CARS), respectively. Serum zonulin levels were significantly higher than healthy controls in children with severe autism. A positive correlation was found between the CARS score, GSRS score and serum zonulin levels (r = ; P < 0.001). Our findings suggest that the severity of gastrointestinal symptoms and severity of autism might be related to increased intestinal permeability in ASDs children.