We carried out a prospective, long-term, combined clinical and MRI follow-up study on 15 patients hospitalized at the Montreal Neurological Institute between 1985 and 1988 with a diagnosis of acute partial transverse myelopathy of unknown etiology. Twelve of the 15 (80%) developed clinically definite or lab-supported definite multiple sclerosis (MS) by the end of a mean follow-up period of 38.5 months. The presence of CNS periventricular white matter lesions by cranial MRI at onset increased the likelihood of development of MS to 93%.