Understandably, the doctor confronted with a patient is under pressure to provide treatment. When, as with cardiac infarction, he knows of no certain treatment, he may feel compelled to translate into specific measures the most tentative suggestions put forward by the clinical research worker. This is why he may prescribe restriction of animal fats and administration of corn oil to a patient after his first attack of cardiac infarction. It may well turn out that this is the most appropriate effective treatment, but the existing evidence is very tenuous. The common belief is that there is a simple progression, leading directly from faulty diet to cardiac infarction. It is said that, in the wealthier countries, many diets have excessive amounts of saturated fats and deficient amounts of unsaturated fats. The result is an abnormally high level of cholesterol in the blood. This leads to atheroma. The coronary arteries consequently tend