Two cases have been presented which illustrate two types of pathology which may be encountered in low grade idiocy with microcephaly. The first case represents a developmental arrest with macrogyria and microgyria interna. In its degree of development, this brain corresponded to the brain of a six months fetus.The second case is one of microcephaly with cystic degeneration of the white matter. History and pathology indicate that the cystic degeneration was due to prolonged asphyxia at birth.Both cases showed micro-hydrocephaly and the difference between this condition and true hydrocephaly has been discussed.Abstract Teaser