Abstract:Autism spectrum disorder (ASD) is a complex neurodevelopmental disorder, with multiple genetic and environmental risk factors. The interplay between genetic and environmental factors has become the subject of intensified research in the last several years. Vitamin D deficiency has recently been proposed as a possible environmental risk factor for ASD. Vitamin D has a unique role in brain homeostasis, embryogenesis and neurodevelopment, immunological modulation (including the brain's immune system), antioxidation, antiapoptosis, neural differentiation and gene regulation. Children with ASD had significantly lower serum levels of 25-hydroxy vitamin D than healthy children.Therefore vitamin D deficiency during pregnancy and early childhood may be an environmental trigger for ASD.