Siblings are reported with severe mental retardation, spastic cerebral palsy and seizures; in addition they had progressive or intermittent jaundice and recurrent infections; they died at 3 and 4 years respectively. Neuropathological studies in one showed a small brain with an almost complete lack of myelin in cerebral white matter, brain stem, cerebellum and anterolateral parts of the spinal cord. The condition most likely represents a dysgenesis of myelin (dysmyelination), possibly due to an inability of oligodendrocytes to form myelin and/or metabolic defects in the process of myelination. This mental retardation condition is probably inherited as an autosomal recessive trait and may represent a special type of a primary CNS developmental defect.