Some investigators have suggested that there are different forms of multiple sclerosis (MS) based on onset age, and that each has a different etiology. 173 Canadian MS patients were matched to controls on age, gender, race and risk zone prior to age 15. Data were collected on: age at onset, gender, initial symptom, disability level, residence history and family background. Three onset age subgroups (early, intermediate and late) were derived. Matched-pair logistic regression analysis indicated that rural residence, use of well water and an MS family history distinguished between patients and controls overall, but showed no significant interaction with onset age. A family history of diabetes distinguished between patients and controls with evidence of age interaction, in that this risk factor decreased in importance as onset age increased.