Cortical biopsies of 13 patients with clinical diagnosis of congenital hydrocephalus, Arnold-Chiari malformation and hydrocephalus, and postmeningitis hydrocephalus were examined by transmission electron microscopy to study the damage of endothelial cells, basement membrane, astrocytic end-feet layer, and perivascular space. Capillaries from the parietal and frontal cortex showed increased vesicular and vacuolar transport, intact endothelial junctions, thin and immature basement membrane, swollen perivascular astrocytic end-feet layer, and enlarged perivascular space. In areas of severe oedema, open endothelial junctions, swollen basement membrane, absent perivascular astrocytic end-feet layer, enlarged perivascular space, and disrupted perivascular neuropil were observed. The electron microscopic findings demonstrated breakdown of the blood-brain barrier in all cases examined.