Five thousand three hundred five World War II and Korean conflict veterans who have been compensated by the Veterans Administration for multiple sclerosis (MS) were matched to controls on the basis of age, date of entry into military service, and branch of service. Case/control ratios for white males, white females, and black males were 1.04, 1.86, and 0.45, respectively. The coterminous 48 states, divided into three tiers on the basis of latitude, exhibited the well-known north-south gradient in risk: For all races and both sexes, case/control ratios were 1.41, 1.00, and 0.53 for the North, Middle, and South tiers. Both white females and black males showed this same north-to-south variation in risk. The case/control ratio for males of races other than black or white was 0.23, with possible deficits in risk for American Indians and Japanese-Americans. Filipinos and Hawaiian Japanese were significantly low-risk groups. These findings suggest that both a racial and a possibly genetic predisposition, as well as a geographically determined differential exposure to an environmental agent, are related to the risk of MS.