We studied 74 children with primitive megalencephaly retrospectively with attention directed to familial megalencephaly, birth history, enlarged pericerebral subarachnoid space (SAS) (idiopathic external hydrocephalus), head and statural growth dynamics, developmental and school prognosis, morphological findings and development of subdural haematoma. In the megalencephalic children, no significant differences were found between those with normal or those with enlarged pericerebral SAS. Out of 62, 31 children (50%) were already megalencephalic at birth. Of 74, 37 children (50%) showed variable degrees of developmental delay which in 18 was transient. Eight out of 74 were mentally retarded. Of 52 children at school age, 42 attend normal schools and 10, of whom 7 are mentally retarded, attend special schools. Three children showed subdural haematoma resulting from apparently minor trauma or occurring spontaneously. We suggest that idiopathic external hydrocephalus represents a variant of primitive megalencephaly with transient increase of intracranial pressure and that it could predispose to the development of idiopathic (spontaneous or non-traumatic) subdural haematoma.