A retrospective study of 40 children with some form of pachygyria was performed at the Children's Memorial Hospital in Chicago. All 40 children had MR brain scans. We analyzed the MR findings, and correlated these findings with the clinical symptoms and course in all the children. We have autopsy findings in 15% these children. Based on our clinical, MR and autopsy findings, in conjunction with the medical literature, we found the following: (1) Pachygyria can occur as an isolated entity without an association with lissencephaly. The MR findings in these children consisted of a brain that demonstrated normal opercularization with either focal or diffuse areas of pachygyria without areas of agyria. These children live longer and have less severity of symptoms than the children with lissencephaly. (2) The MR findings in children with lissencephaly consisted of a brain that demonstrated abnormal opercularization with areas of total agyria or areas of agyria with pachygyria. (3) The MR findings in 25% of our children with polymicrogyria simulated pachygyria. The MR findings of the brain in these children consisted of a 'nubby' appearance to the outer surface of these abnormal gyri which resembled pachygyria but on histologic exam was polymicrogyria.