Recently, we described an as yet unidentified white matter disorder in eight patients which probably has an autosomal recessive mode of inheritance. The disease was characterized by megalencephaly and leukoencephalopathy with onset during the 1st year of life and a delayed onset of slowly progressive neurological dysfunction. No basic biochemical defect could be identified. Brain biopsy was performed in one of the patients and the histopathological findings are described in the present report. A spongiform leukoencephalopathy was revealed, without cortical involvement. Most vacuoles were covered by single five-layered membranes, representing single myelin lamellae. Some vacuoles were partially covered by multi-lamellar myelin sheaths or oligodendroglial cell extensions. The vacuoles were never found in the middle or inner parts of myelin sheaths, but involved the outermost lamellae of myelin sheaths only, whereas the remainder of the myelin sheaths remained undisturbed. The histopathological findings place the disease among the vacuolating myelinopathies, although it is distinct from the well-known forms. Possible pathophysiological mechanisms are splitting of the outermost myelin lamellae at the intraperiod line or a disturbance of compaction of the outermost myelin lamellae at the intraperiod line.