Hemi-hydranencephaly is a very rare condition characterized by complete or almost near-complete unilateral absence of the cortical cortex, which is filled by a sac of cerebrospinal fluid. Prenatal vascular disruption with occlusion of the carotid artery territories ipsilateral to the damaged brain is the presumed pathogenesis.We have selected nine cases that fit the clinical and pathologic characteristics of hemi-hydranencephaly, demonstrating that destruction of one hemisphere may be not always associated with severe neurologic impairment and may allow an almost normal life. This disorder is an example of a possible prenatal re-organization in which the right and left cerebral hemispheres present functional potentiality to make up the damaged brain.The cases reported in the literature are discussed, including a patient previously reported and followed-up for 10 years. A review of the cases is performed with an evaluation of the most important aspect of this rare and mysterious disorder.