Between 1949 and 1984, 150 multiple sclerosis patients consumed low-fat diets. Fats, oils, and protein intakes; disability; and deaths were determined. On daily fat consumption of less than 20.1 g (average 17 g), 31% died and deterioration was slight. Daily intakes of greater than 20 g (average of either 25 or 41 g) were attended by serious disability and deaths of 79% and 81%, respectively. Oil intakes bore an indirect relationship to fat consumption. Treatment early and before severe disability developed improved prognosis, and females tended to do better than males. High sensitivity to fats suggests they are involved in the genesis of multiple sclerosis.