Ecologic correlations suggest that higher intake of saturated fat and lower intake of polyunsaturated fat might increase the risk of multiple sclerosis (MS), but the results of case-control studies have been inconsistent. Because no prospective data are available, the authors examined these associations in two large cohorts, the Nurses' Health Study, which consisted of 92,422 women with 14 years of follow-up (1980-1994) and the Nurses' Health Study II, which consisted of 95,389 women with 4 years of follow-up (1991-1995). They documented 195 new cases of MS. The pooled multivariate relative risks comparing women in the highest quintile with those in the lowest were 1.1 (95% confidence interval: 0.7, 1.7) for total fat, 0.7 (95% confidence interval: 0.5, 1.2) for animal fat, 1.2 (95% confidence interval: 0.7, 2.1) for vegetable fat, 0.8 (95% confidence interval: 0.5, 1.3) for saturated fat, 1.1 (95% confidence interval: 0.7, 1.7) for monounsaturated fat, 1.7 (95% confidence interval 1.0, 2.8) for n-6 polyunsaturated fat, 1.3 (95% confidence interval: 0.8, 2.0) for trans unsaturated fat, and 0.7 (95% confidence interval: 0.4, 1.1) for cholesterol. Omega-3 fatty acids from fish were also unrelated to risk. However, the authors observed a nonsignificantly lower risk of MS for a higher intake of linolenic acid. These findings do not support relations between intakes of total fat or major specific types of fat and the risk of MS.