Hydranencephaly is a condition in which cerebral hemispheres are absent and reduced to fluid-filled sacs in a normal skull. Numerous causes have been proposed. We report a male infant with hydranencephaly and congenital vascular malformations (port wine stains, generalized nevus flammeus, anomalous retinal vessels, and absent internal carotid flow). Magnetic resonance imaging of the brain showed absence of most of the cerebrum except for small portions of the occipital cortex and thalami. Magnetic resonance angiography showed flow within the vertebral and basilar arteries without internal carotid intracranial flow above the internal carotid petrous and cavernous portion. This is a report of cutaneous and retinal malformations associated with hydranencephaly. Vascular malformations of larger vessels (e.g., webbing of the carotid arteries and an absent internal carotid arterial system) have been observed in other infants with hydranencephaly, and are proposed to lead to brain destruction. The case reported herein supports the role of primary vascular malformations in the development of some cases of hydranencephaly. Copyright 2001 Wiley-Liss, Inc.