A progressive neurological condition characterised by megalencephaly, spasticity, ataxia and seizures in two siblings of consanguineous parents is described. The electroencephalogram showed posterior discharges and an unusual photoparoxysmal response whereas brainstem auditory evoked potential findings were consistent with a white matter disorder. Computerized tomography scans revealed diffuse hypodensity of the white matter and a brain biopsy on one sibling showed features of dysmyelination without evidence of demyelination, Rosenthal fibres or the spongy changes characteristic of Canavan's disease. There was no detectable biochemical abnormality. This combination of clinical, neurophysiological and neuropathological abnormalities has not previously been described.