Preventive trials and the "diet-heart" question: wait for results or act now.

Is the relationship between coronary heart disease risk factors and the risk of disease causal? Direct evidence would have to come, of necessity, from preventive trials. However, the difficult nature of such trials carries the inherent danger of false negative results; even positive results may be open to challenge. With regard to demonstrating specifically a preventive effect of lowering serum lipid levels through nutritional changes alone, this evidence may never be forthcoming since ongoing trials intervene simultaneously on multiple factors; against this disadvantage, the strengths of such trials are reviewed. The view is stated and discussed that preventive action in the community is justified now, without waiting for the results of preventive trials, however important they will be eventually to help the development of optimally effective programs.