Patients with chronic myelopathy of unknown origin were separated into two groups on the basis of presence or absence of oligoclonal immunoglobulin G (lgG) in cerebrospinal fluid (CSF). Thirty-nine of 48 patients (81%) with oligoclonal lgG in CSF had measles virus antibodies in the CSF and 31 (65%) showed reduced serum/CSF measles virus antibody ratios, in comparison with the corresponding ratios of adenovirus, group-specific, penton hemagglutination-enhancement and poliovirus neutralization-enhancement antibodies. Of 25 patients with myelopathy, but without oligoclonal lgG in their CSF, three had detectable titers of measles CSF antibodies and one of these had a greatly reduced serum/CSF ratio. The conditions of patients with chronic myelopathy of unknown origin and oligoclonal lgG in CSF may be diagnosed as probable multiple sclerosis (MS), in contrast to patients with this disease who lack oligoclonal lgG IN CSF. However, the clinical features of the disease in these two groups do not differ substantially.