The bilateral frontoparietal polymicrogyria syndrome is characterized anatomically by predominantly frontoparietal diffuse polymicrogyria and clinically by generalized tonic-clonic seizures and mental retardation developing in early childhood. Despite the diffuse nature of the cortical involvement, patients do gain the ability to speak and to walk. In addition, epilepsy can be relatively well controlled with antiepileptic drugs. In three cases of bilateral frontoparietal polymicrogyria, the patients demonstrated consistently similar clinical and neuroimaging characteristics, with more severe structural and developmental abnormalities than those reported previously. The designation proposed here recognizes such cases as a severe form of bilateral frontoparietal polymicrogyria. An ethnic difference or a causal gene other than GPR56 might be responsible for the difference in severity between the classical and the more severe forms of bilateral frontoparietal polymicrogyria.