Age at onset of multiple sclerosis (MS) symptoms was ascertained for subsets of some 4,400 veterans of World War II who had been adjudged ''service-connected'' for this condition. Average age at onset was 27.0 years for white men, 27.7 for white women, and 27.5 for black men. The unexpectedly older age for women is attributed to their older age at entry into service. When the coterminous United States was divided into three horizontal tiers of states, we found a strong effect of geography on age at onset. By state of residence at entry into active duty (EAD), white men had an average age at onset of 26.4 years in the northern tier, 27.3 years in the middle, and 28.8 years in the south. Trends were similar for white women and black men. Migrants, defined as those whose birth and EAD tiers differed, showed increasing ages at onset with southward moves. A statistical model used to discriminate between the influence of birth and EAD tiers on age at onset confirmed the significant effect of EAD alone. These data are compatible with the theses that the cause of MS is less common (or less efficient) in locations where the clinical disease is less common, and that its acquisition therefore occurs at an older age in those locales.