The use of magnetic resonance imaging (MRI) has resulted in the detection of an increasing number of children with an apparently leukodystrophic white matter. Laboratory tests and the clinical presentation, however, often do not correspond to any known entity and the course is sometimes not progressively deteriorating. Such children with white-matter changes and no known diagnosis were the subject of this Swedish multicentre study, in which MRI findings and clinical data from 100 children considered to have white-matter abnormalities were assessed during the period 1992-1995. At re-evaluation of MR images by an established "white-matter group" of neuroradiologists, paediatric neurologists, neurologists and neurochemists, the MRI signal of the white matter was considered normal in eleven children and eleven had mainly a grey matter affection. Of the remaining 78 children with white matter abnormalities, a diagnosis was found in 32, but in 46 children no diagnosis could be established. A progressive downhill course characterised 17, probably representing hitherto undefined types of leukodystrophies. Five children had a relapsing-remitting course, and in 11 it was difficult to establish whether the course was progressive or stationary. The disease was non-progressive in 13. This group of non-leukodystrophic white-matter changes obviously represents maldevelopments of myelin formation, thus dys- or hypomyelination rather than demyelination.