The leukodystrophies are degenerative diseases that involve primarily the white matter of the brain. The most common leukodystrophies result from known disturbances in the synthesis or catabolism of myelin such as a block in the catabolism of sulphatides and of galactocerebrosides, respectively, in metachromatic leukodystrophy and in Krabbe disease, or from synthesis of an abnormal proteolipid protein in Pelizaeus-Merzbacher disease. The cause of white matter involvement in other leukodystrophies remains unknown even though metabolic anomalies, such as accumulation of acetylaspartic acid in Canavan disease, have been demonstrated. Common clinical features of the leukodystrophies include neurological deterioration following a period of normal development, predominant involvement of motor function at least initially, and absence of convulsions or myoclonus. Imaging-especially magnetic resonance-shows changes in density or signal from central white matter. Most leukodystrophies feature suggestive symptoms and signs such as effects on peripheral nerves' myelin in Krabbe disease and metachromatic leukodystrophy, or X-linked inheritance and slow deterioration in Pelizaeus-Merzbacher disease. Therapy of the leukodystrophies is purely symptomatic in most cases. Trials of bone marrow transplantation are being pursued for metachromatic leukodystrophy and adrenoleukodystrophy.