Smith-Kingsmore syndrome is a neurological disorder characterized by a head that is larger than normal (macrocephaly), intellectual disability, and seizures. In some people with this condition, the ability to speak is delayed or never develops. Some children with Smith-Kingsmore syndrome have neurodevelopmental conditions known as attention-deficit/hyperactivity disorder (ADHD) and autism spectrum disorder, which is characterized by impaired communication and social interaction. Structural brain abnormalities may also be present in affected individuals. For example, one or both sides of the brain may be enlarged (hemimegalencephaly or megalencephaly) or have too many ridges on the surface (polymicrogyria), or the fluid-filled spaces near the center of the brain (ventricles) may be bigger than normal (ventriculomegaly).