Alexander's disease, a rare and fatal disorder of the central nervous system, most commonly affects infants and young children but can also occur in older children and sometimes adults. In infants and young children, it causes developmental delay, psychomotor retardation, paraparesis, feeding problems, usually megalencephaly, often seizures, and sometimes hydrocephalus. Juvenile cases often do not have megalencephaly and tend to have predominant pseudobulbar and bulbar signs. In both groups, characteristic magnetic resonance imaging findings have been described. In adult cases, the signs are variable, can resemble multiple sclerosis, and might include palatal myoclonus. In all cases, the examination of brain tissue shows the presence of widely distributed Rosenthal fibers. Almost all cases have recently been found to have a heterozygous, missense, point mutation in the gene for glial fibrillary acidic protein, which provides a new diagnostic tool. In most cases, the mutation appears to occur de novo, not being present in either parent, but some adult cases are familial.