Bilateral periventricular nodular heterotopia (BPNH) is a neuronal migration disorder, characterized by grey-matter cellular rests in the periventricular regions. Patients frequently present with seizures during childhood or later in life. Monozygotic twin sisters were found to have enlarged head circumferences (>95th centile) during a routine medical visit at 6 months of age. A brain CT scan revealed grey-matter cellular rests in the subependymal regions extending to the subcortical white matter. In some areas these cell aggregates had nodular margins, consistent with the diagnosis of BPNH. At 6 years of age they are cognitively and neurologically normal. They have not had any seizures and their heads continue to grow along the 95th centile.