Numerous studies have indicated an association between vitamin D deficiency and the immune dysregulation and the pathogenesis of autoimmunity. An increased prevalence of vitamin D deficiency has been demonstrated in several pediatric autoimmune diseases, including autoimmune thyroid disease and Type-1 diabetes mellitus. This study was undertaken to determine the prevalence of vitamin D deficiency in children with PANS, an autoimmune disorder characterized by abrupt-onset neuropsychiatric symptoms.