Abstract.It may be hypothesized that many diseases are associated with an overproduction of eicosanoids from the n‐6 acid, arachidonic acid (20:4n‐6), and the formation and function of these n‐6 eicosanoids can be antagonized by dietary n‐3 fats. This hypothesis provides a basis for evaluating the benefits and risks of including various amounts of n‐3 and n‐6 fats in the diet. Understanding the impact of dietary polyunsaturated fats leads inevitably to a reappraisal of what is ‘normal’ in terms of what is typical and what is desirable for the fatty acid composition of tissue lipids, the magnitude of eicosanoid‐mediated responses, and the frequency and severity of