Lissencephaly is a brain malformation manifested by a smooth cerebral surface and caused by incomplete neuronal migration. Clinical sequellae include minor craniofacial changes (bitemporal hollowing, small jaw), severe mental retardation, and other neurological abnormalities. Patients with classical or type I lissencephaly and its sequellae but no other significant anomalies are classified as having isolated lissencephaly sequence. Possible causes of isolated lissencephaly sequence include ischemia or viral infection during the time of neuronal migration, microdeletion within the Miller-Dieker syndrome critical region in chromosome band 17p13.3, and Mendelian inheritance. The last is based on a report of a single family with three affected children in 1933. We report four patients with isolated lissencephaly sequence from two unrelated families who provide further support for autosomal (or possibly X-linked ) recessive inheritance. In the first family, three brothers were affected. In the second, the parents are first cousins.