X-linked lissencephaly with absent corpus callosum and ambiguous genitalia is a newly recognized syndrome responsible for a severe neurological disorder of neonatal onset in boys. Based on the observations of 3 new cases, we confirm the phenotype in affected boys, describe additional MRI findings, report the neuropathological data, and show that carrier females may exhibit neurological and magnetic resonance imaging abnormalities. In affected boys, consistent clinical features of X-linked lissencephaly with absent corpus callosum and ambiguous genitalia are intractable epilepsy of neonatal onset, severe hypotonia, poor responsiveness, genital abnormalities, and early death. On magnetic resonance imaging, a gyration defect consisting of anterior pachygyria and posterior agyria with a moderately thickened brain cortex, dysplastic basal ganglia and complete agenesis of the corpus callosum are consistently found. Neuropathological examination of the brain shows a trilayered cortex containing exclusively pyramidal neurons, a neuronal migration defect, a disorganization of the basal ganglia, and gliotic and spongy white matter. Finally, females related to affected boys may have mental retardation and epilepsy, and they often display agenesis of the corpus callosum. These findings expand the phenotype of X-linked lissencephaly with absent corpus callosum and ambiguous genitalia, may help in the detection of carrier females in affected families, and give arguments for a semidominant X-linked mode of inheritance.