An investigation of the occurence of multiple sclerosis (MS) was undertaken in the City of Galion, Ohio, USA, because of a report of an increased number of cases. As of June 1,1987, there were 18 living cases of MS in Galion and Polk Township, for a prevalence rate of 112 cases per 100,000 population. The expected rate is approximately 65–170 cases per 100,000. In a case-control study, residents of Galion or Polk Township who had MS were compared to residents who did not have MS. The controls were matched to the cases on age and sex and had lived in Galion for at least as long as their matched case. The cases and controls did not differ in the distribution of their present or past Galion addresses, occupational histories or workplace exposures. Cases were more likely to have graduated from high school and college than controls. Cases were more likely than controls to report a history of allergies, to recall two or more relatives who had neurologic diseases that began before their first MS symptoms, to report owning a cat that died of unexplained causes and to recall having received oral polio vaccine. Cases and controls had similar levels of antibodies to measles, chickenpox, cytomegalovirus and the human T-cell lymphotrophic virus