The myelin membrane is essential for rapid conduction of nerve impulses through the central nervous system. Failure of myelination--dysmyelination--may arise through several mechanisms. The synthesis of a particular myelin protein can be defective, as occurs for proteolipid protein in Pelizaeus-Merzbacher disease and for myelin basic protein in the 18q- syndrome. Delay in myelination with a more generalized diminution in white matter is characteristic of many inherited metabolic diseases, including galactosemia, pyridoxine-dependent seizure disorder, glutaric aciduria type 1, and infantile Refsum disease. Demyelination or breakdown in myelin is characteristic of metachromatic leukodystrophy, Krabbe disease, mitochondrial disorders, adrenoleukodystrophy, Canavan disease, Alexander disease, and orthochromatic leukodystrophy. A fourth category is reserved for malformation syndromes. These include Cockayne, Fukuyama, Walker-Warburg, and Angelman syndromes. Demyelination also occurs in HIV-infected individuals with central nervous system findings and in multiple sclerosis. Much of the evidence for leukodystrophy in these disorders comes from neuroimaging. Some of these disorders are treatable.