The authors examined the relation between 24-hour dietary fiber intake at baseline survey in 1972-1974 and subsequent 12-year ischemic heart disease mortality in a southern Californian population-based cohort of 859 men and women aged 50-79 years. Relative risks of ischemic heart disease mortality in those with dietary fiber intake of 16 gm/24 hours or more compared with those with intake less than 16 gm/24 hours were 0.33 in men and 0.37 in women. A 6 gm increment in daily fiber intake was associated with a 25% reduction in ischemic heart disease mortality (p less than 0.01). This effect was independent of other dietary variables, including calories, fat, cholesterol, protein, carbohydrate, alcohol, calcium, and potassium. Some, but not all, of this effect appears to be mediated through the known cardiovascular risk factors: after multivariate adjustment for age, sex, blood pressure, plasma cholesterol, obesity, fasting plasma glucose, and cigarette smoking habit, the magnitude of the protective effect of fiber was reduced but still significant in both sexes combined. These findings support the hypothesis that high dietary fiber intake is protective for ischemic heart disease mortality.