Although dietary trans fatty acids can effect the fatty acid composition of tissues, they are not considered to be harmful, provided sufficient essential fatty acids are present in the diet. 2. A reduction in dietary saturated fatty acids (SFA) and dietary increases in mono- and polyunsaturated (PUFA) fatty acids are beneficial in reducing the risk of coronary heart disease. 3. There is only limited evidence of a beneficial effect on hypertension by reducing either total fat or SFA in the diet. 4. There is a little evidence of a role for dietary fat in the initiation of cancer but linoleic acid has been implicated in tumor promotion and chemical carcinogenesis. 5. Dietary PUFA might be beneficial in the treatment of certain autoimmune diseases such a multiple sclerosis.