Lissencephaly (agyria or smooth brain) is a rare congenital anomaly representing an arrest of brain development before the third or fourth month of gestational age. It may occur alone or in association with many other syndromes. In analyzing the findings in four patients it is evident that the main computed tomographic and magnetic resonance features are: (a) smooth brain surface, (b) shallow sylvian fissures resulting in a figure-eight appearance of the axial brain sections, (c) decreased white matter and a thick brain cortex, (d) absent or severely attenuated grey-white matter interdigitations, and (e) dilatation of the lateral ventricles. Other associated anomalies include microcephaly, absent corpus callosum, neuronal heterotopia, and cerebellar hypoplasia. Pachygyria is a less severe anomaly occurring at a later stage of brain development and is characterized by the presence of a relatively few broad coarse gyri. Lissencephaly and pachygria may coexist in the same brain.