Brain MRI was performed on 12 patients with acute disseminated encephalomyelitis (ADEM). Multifocal white matter lesions indistinguishable from those seen in multiple sclerosis (MS) were found in 10. In 5 there were rather extensive symmetric abnormalities in the cerebral (2) or cerebellar white matter (2), or basal ganglia (1). Follow-up MRI after intervals of 2 wks to 18 months demonstrated partial resolution of the abnormalities, but some persisting lesions. New MRI lesions were found at follow-up in only 1 case after an interval of 2 wks. Serial MRI makes a useful contribution to the distinction between MS and ADEM.