Post infectious encephalomyelitis and multiple sclerosis are both inflammatory demyelinating disorders of the central nervous system. Whereas multiple sclerosis is a multi phasic disease with recurrent episodes disseminated in time and place, post infectious encephalomyelitis is usually considered to be a monophasic illness. This study used serial brain MRI to clarify whether the latter hypothesis holds for the long term. Post infectious encephalomyelitis was defined as the development of a central nervous system white matter disorder occurring in close temporal relationship with a viral, bacterial or other infection. There were eleven patients, mean age at presentation 21 years (4-48), and mean period of follow-up of 8 years (3.5-11). T2-weighted brain MRI was abnormal in all 11 cases during the acute stages of the illness. On follow-up 6 patients had made a complete clinical recovery, 4 patients had mild residual deficits and one severe neurological deficits necessitating ventilatory support. No patient experienced an exacerbation during the follow-up period. MRI revealed complete resolution of abnormalities in 3 and partial resolution in 7; new white matter lesions were seen in only one patient. This long term follow-up study suggests that there is a definable group with post infectious encephalomyelitis who exhibit a monophasic clinical and MRI pattern in the long term.