To clarify the clinicopathological features of X-linked hydrocephalus, the authors studied 30 affected males from 15 families. In utero ultrasonography, performed at 21 to 40 weeks of gestation, revealed 18 fetuses with hydrocephalus. Computerized tomography (CT) revealed bilateral enlargement of the lateral ventricle with preponderant dilation of the posterior horn. In five patients with complete magnetic resonance (MR) imaging data, the most specific finding was localized atrophy of the anterior vermian lobe. Other MR imaging findings included a large massa intermedia, flat corpora quadrigemina, a small brainstem, and diffuse hypoplasia of the cerebral white matter. In all cases, the corpus callosum was hypoplastic or aplastic. The aqueduct was patent in four of five cases. Asymmetrical reduction of the ventricular size and a rippled ventricular wall were characteristic postshunt CT findings. Progressive macrocephaly and symptoms due to increased intracranial pressure were ameliorated by the shunt; however, the neurological outcome was not improved by shunting. Of 14 patients who lived to be between 2 and 18 years of age, all are retarded. These results indicate that X-linked hydrocephalus is not a disease of simple ventriculomegaly due to aqueduct stenosis alone but involves other complicated central nervous system anomalies.