The premise that measures used to lower the plasma lipids in patients with hyperlipidemia will lead to reductions in new events of coronary heart disease (the Lipid Hypothesis) should be reconsidered today as a result of several recent reports of large-scale double-blind drug trials in the United Kingdom and in the United States. To that end, the published evidence that bears on tests of the hypothesis by dietary and drug interventions is reviewed, and the conclusion reached that the hypothesis has not yet been adequately tested. A phased program is described that will prepare the ground for a fuller and more definitive trial of the premise in the future: the first steps must be to establish that a combined diet/drug regimen in large numbers of adult male hyperlipidemic patients is acceptable and essentially harmless and that during an observation period of several years a high rate of adherence to the regimen can be attained. Any advice to the general public to make large dietary changes now is considered premature.