A group of 814 men at large, 40 to 59 years old, have been placed in a diet relatively rich in polyunsaturated fatty acids. The study diet has significantly lowered the serum cholesterol levels and maintained these lower levels for periods as long as five years. The study protocol has also been effective in significantly reducing the incidence of obesity and hypertension during the first four years of study participation. Among a control group of 463 men of similar age, the prevalence of these conditions was stable. After the men had participated in the study for four years, the differences between the experimental and control groups in prevelance of these risk factors (obesity, hypertension, and hypercholesterolemia) were statistically significant. Accompanying these significant differences, a statistically significant difference was observed between the two groups in morbidity from new coronary heart disease.