Intermediate (8--9 nm) filaments of human central nervous system astrocytes were isolated from the gliosed white matter of cases of adrenoleukodystrophy (ALD). This hereditary lipidosis is characterized pathologically by demyelination, loss of axons, and replacement of the white matter of the caudal cerebrum by a glial scar. Glial filaments were composed largely of a single protein component with a mol wt of about 49,000 daltons. Smaller components (44,000--39,000 daltons) were detected in some samples, and appear to represent degradation products of the filament protein. Human neurofilaments were isolated from the normal frontal white matter of ALD cases by the standard myelin-free axon technique. Isolated glial and neurofilament proteins comigrated during acrylamide gel electrophoresis in SDS. Polypeptides resulting from cyanogen bromide cleavage of the two filament proteins were the same. Both proteins reacted with rabbit antisera raised against isolated bovine neurofilament protein and human glial fibrillary acidic protein.