Epidemiology of multiple sclerosis in US veterans: III. Migration and the risk of MS.

World War II or Korean Conflict veterans with MS (5,305 in number) and pre-illness-matched controls were compared for residence at birth and entry on active duty (EAD) within three north-south tiers of states in the United States. A strong north-south gradient of MS risk was present. Migrants were defined as those whose birth and EAD tier differed. For white men of World War II, all white men, and all whites, there were highly significant reductions in risk for moves southward from either the north or middle tier, and increases in risk for moves northward from the middle tier. Increases similar in magnitude of middle to north did not attain statistical significance in the few southern-born migrants. For the small groups of black men and white men of Korean service, trends were similar but did not attain significance, whereas for white women, they were of borderline significance. Findings imply an environmental cause for MS, with acquisition years before symptom-onset.