Reducing the intake of saturated fatty acids reduces the risk of coronary disease. This paper reviews the merits of two types of diets reduced in saturated fat. Low-fat, high-carbohydrate diets lower plasma low-density lipoprotein (LDL) but also lower high-density lipoprotein (HDL) concentrations and raise plasma very-low-density lipoprotein. The predicted net effect on coronary risk is zero. Weight loss with low-fat diets is modest and insufficient to offset the fall in HDL. Evidence for other beneficial effects of low-fat diets in incomplete. In contrast, diets low in saturated fat but high in unsaturated oils improve the ratio of HDL to LDL in plasma and thus reduce the predicted coronary risk. Recommendations to reduce total fat intake are therefore too imprecise; guidelines should aim specifically at saturated and probably also at trans fatty acids, whereas recommendations for restriction of cis-unsaturated fatty acids are not supported by firm scientific evidence.