Twenty-six years after a total right hemispherectomy for tumor when thirteen years old, a right-handed patient presented with disturbances of equilibrium, urinary incontinence, and a marked reduction in motor impulses, in spite of a subnormal intelligence quotient (0.85). Computed tomography examinations suggested the presence of a normal pressure hydrocephalus. However intracranial pressure monitoring and isotopic and cerebrospinal fluid perfusion tests showed the absence of any cerebral obstruction, and free communication between the cerebral and spinal spaces, though intracranial volume capacity accommodation was greatly reduced. Clinical examination demonstrated good quality sensory appreciation by the left half of the body, the possibility of detecting an object in the visual hemifield supposedly blind, and perfect location of a sound source. Somesthetic evoked potentials were studied by stimulating the median nerve in the left and right wrist regions. Left hemisphere evoked potentials from a contralateral stimulus, were of normal latency, amplitude and morphology, the response to an ipsilateral stimulus also presenting an early potential component (N 26 starting at N 21). This exceptional finding in a hemispherectomized patient could be related to activity of the spinoreticulothalamic tracts in the lemniscal or extralemniscal systems. Auditory function tests demonstrated perfect stereoaudiometry and the quality of perception during dichotic listening tests. Auditory evoked potentials in the brain stem included only minor functional-type anomalies, with slightly increased latencies in the pons on the left, and the mesencephalon and pons on the right. The presence of a bilateral wave VI was related to activity of the median geniculate pathway. Visual function tests showed good quality left photomotor reflexes and positive object detection in the anopsic hemifield. Results of these tests were reproducible during the same examination and after an interval of several months. The relevant literature is reviewed.