The evidence for recommending a reduction of one third or more of dietary fat as a very likely means of reducing coronary heart disease in affluent nations is far from complete. That it will do no harm is an unacceptable scientific argument, and it is still up to the proponents of this policy to prove its long-term safety. I am persuaded, because the facts are very much stronger, that such a diet should be recommended for those who have atherogenic lipoproteins in the top quintile of their distribution. But even in such people, too much should not be expected or promised. What we really need to know is how dietary saturated fat leads to the development of coronary heart disease as well as coronary atherosclerosis; why it may do so in some and not in others; and much more about the effects of specific dietary fatty acids on the mechanisms involved.