The diagnostic features and clinical signs of 21 patients with lissencephaly type I are reviewed from the Department of Neurology, Westeinde Hospital, The Hague, The Netherlands; the Departments of Child Neurology, Academic Medical Centre, Amsterdam, and Sophia Children’s Hospital, Rotterdam. A multicenter study was conducted with the cooperation of all child neurology departments in The Netherlands. Lissencephaly was diagnosed at autopsy in two patients and by CT scan in 19. The criteria of Dobyns WB (1987) were used for the classification of Lissencephaly: Type I Isolated lissencephaly sequence (ILS), Miller-Dieker syndrome (MDS); Type II, Walker-Warburg syndrome (WWS), Fukuyama congenital muscular dystrophy; Rare forms, Neu-Laxova syndrome (NLS), Cerebro-cerebellar syndrome (CCS). Lissencephaly type I is classified according to pathology and radiology as follows: Grade 1, complete agyria; Grade 2, agyria with some sulci; Grade 3, a mixture of about 50% agyria and 50% pachygyria; Grade 4, complete pachygyria. Of 21 patient with lissencephaly type I, 17 had isolated lissencephaly and 4 had the Miller-Dieker syndrome. More severe abnormalities in gross brain morphology occurred in MDS than in ILS. Facial dysmorphism was most frequent in MDS patients and microcephaly in combination with facial abnormalities increases the suspicion of MDS. All children with lissencephaly in this study were severely retarded, 86% developed epilepsy before the age of six months, and one-third had infantile spasms. The somatic signs of 19 patients with chromosome deletion 17p are described for the present series of patients and those in the literature: malformed fingers, congenital heart-defect, sacral dimple, cryptorchidism, and malformed kidneys. 
COMMENT. The diagnosis of lissencephaly is made by clinical and neuroradiological findings. Cases of isolated lissencephaly may be distinguished from the Miller-Dieker syndrome without the results of chromosome analysis in most cases. The CT signs of lissencephaly include smooth brain surface and no Sylvian fissure with figure 8 appearance, sharp demarcation between gray and white matter, and colpocephaly. Facial dysmorphism may show high forehead, bitemporal hollowing, and micrognathia.