The aim of this thesis was to assess the effect of different of intake of polyunsaturated fatty acids on indices of oxidative stress, particularly damage to lipids and DNA and to examine whether this effect could be modified by supplementation with the lipid soluble antioxidant, vitamin E. This was assessed using a split plot/change over dietary study, where half and volunteers consumed a diet containing 5% PUFA (low PUFA) as food energy for 4 weeks and after a washout period of up to 10 weeks, consumed a 15% PUFA (high PUFA) diet for another 4 weeks. The other volunteers completed this protocol in reverse. Total fat, carbohydrates and protein, and vitamins E and C contents of the diets were constant and they were provided either with or without an additional 80mg d-tocopherol acetate/day. The results showed that although plasma total cholesterol showed a small significant increase after the low PUFA diets there was no change after consumption of the high PUFA diets or either of the vitamin E supplemented diets. Indices of oxidative stress (whole blood oxidised glutathione and urinary thiobarbituric acid reactive substances (TBARS)) were increased following consumption of the high PUFA diet. However, there was no change in non-specific plasma indices of lipid peroxidation, conjugated dienes and TBARS, nor in red cell antioxidant enzymes, glutathione reductase, and catalase. These results indicate that increasing dietary levels of PUFA may adversely affect some indices of lipid peroxidation, DNA damage and antioxidant status whilst not appearing to improve lipoprotein profiles. Increasing the dietary intake of vitamin E appears to ameliorate the potentially damaging effects of PUFA. Therefore, care should be taken when providing dietary advice on PUFA intake and an adequate intake of antioxidants to match any increased PUFA may be important for preventing oxidative stress.