平脑症（lissencephaly） 是一种先天性的脑部缺陷病。为希腊文"lissos"（平滑的）和"encephalos"（大脑）的结合。

在正常人的大脑中，皮质会有脑廻，其为膨隆而弯曲的折叠面。而平脑症的脑部并不会出现脑廻或很少量的脑回，而造成在外观上产生平滑的脑部。

平脑症的幼童通常无法存活超过2岁。部分幼童存活下来，但脑的发展只有3至5个月大的程度。

Lissencephaly, which literally means smooth brain, is a rare brain formation disorder caused by defective neuronal migration during the 12th to 24th weeks of gestation resulting in a lack of development of brain folds (gyri) and grooves (sulci). It is a form of cephalic disorder. Terms such as 'agyria' (no gyri) or 'pachygyria' (broad gyri) are used to describe the appearance of the surface of the brain. Children with lissencephaly generally have significant developmental delays, but these vary greatly from child to child depending on the degree of brain malformation and seizure control. Life expectancy can be shortened, generally due to respiratory problems.