Hemagglutination inhibition (HI) antibody titers to measles, rubella, mumps, and La Crosse virus in 137 patients with multiple sclerosis and 137 controls matched by date of birth and place of residence were measured and are reported according to age. The measles HI titers were significantly higher in serum specimens of patients with multiple sclerosis (geometric mean 1/82) than in the controls (geometric mean 1/43). In addition, the measles HI titers decreased significantly in the older age groups of the controls, whereas a similar decrease was not observed in the MS patients. No significant differences in the HI titers of rubella, mumps, and La Crosse virus were found between the MS group and the control group. Measles antibodies were further analyzed in a representative smaller series of sera from 49 MS patients and 49 controls by inhibition of salt-dependent hemagglutinin, gel precipitation, and platelet aggregation. With each of the techniques used, higher levels of measles antibody titers or stronger reactions were observed in the MS group. However, when measles antibodies detected with various techniques were compared in individual serum specimens of MS patients, no clear correlation between the high titers or strong reactions was discernible. The nature of the antibody response to measles virus in MS patients and the light thrown by the results obtained on the etiopathogenesis of multiple sclerosis is discussed.