Between 1949 and 1984, 150 patients with multiple sclerosis consumed low-fat diets. Fat, oil, and protein intakes; disability; and deaths were determined. With a daily fat consumption less than 20.1 g/day (av 17 g/day), 31% died, and average deterioration was slight. A daily intake greater than 20 g/day (av 25 or 41 g/day) was attended by serious disability and the deaths of 79 and 81%, respectively. Oil intake bore an indirect relationship to fat consumption. Minimally disabled patients who followed diet recommendations deteriorated little if at all, and only 5% failed to survive the 34 yr of the study, whereas 80% who failed to follow diet recommendations did not survive the study period. The moderately disabled and severely disabled patients who followed diet recommendations carefully did far better than those who failed to follow the diet. In general, women tended to do better than men. Those patients treated early did better than those in whom treatment was delayed. High sensitivity to fats suggests that saturated animal fats are directly involved in the genesis of multiple sclerosis.