The first reported case of lissencephaly resulting from a consanguinous union strengthens the supposition that in some cases, it is transmitted as an autosomal recessive trait. Comparison of this case with a sporadically occurring case of lissencephaly, with different cortical morphology, suggests that lissencephaly may be an example of either varying gene expressivity or genetic heterogeneity. Lissencephaly and pachygyria may eventually be shown to be due to different causes, some inherited, some acquired. The classical examples of lissencephaly are different morphologically from a case in which antenatal cytomegalovirus infection had produced a small smooth brain. This suggests that antenatal viral infections are destructive rather than teratogenic.