The vegetarians in Taiwan consume diets high in polyunsaturated fatty acids. To investigate whether this dietary pattern results in high susceptibility of LDL to oxidation, 109 long-term (8 +/- 5 y) male and female vegans and lactovegetarians (ages 31-45 y) from Taipei and females from Hualien and matched omnivores were recruited to have 24-h-recall dietary assessments and blood lipid analysis. Body mass index and blood pressure were significantly lower in all vegetarian groups than in the matched omnivore groups (P < 0.05). Vegetarians consumed less energy except in the males and less protein, fat and cholesterol (P < 0.05). The mean polyunsaturated/saturated fatty acid (P/S) ratio of 2.4 in vegetarian diet was about two times that in omnivore diet (P < 0. 001). The concentrations of plasma total- and LDL-cholesterol (LDL-C) but not HDL-cholesterol (HDL-C) were significantly lower (P < 0.001) and resulting HDL-C/LDL-C ratio was 38, 46 and 30% higher (P < 0.01) in Taipei female, male and Hualien female vegetarians, respectively, than in the matched omnivores. Plasma triglyceride concentration was significantly lower only in the Hualien women vegetarians (31%, P < 0.001) than in the matched omnivores. The lag time of conjugated diene formation in LDL oxidized in vitro induced by copper was longer in Taipei female (62%, P < 0.001), male (29%, P < 0.05) and Hualien female (38%, P < 0.01), and the production of thiobarbituric acid reactive substances (TBARS) in LDL after 2-4 h of oxidation was 22-32% less (P < 0.005) in Taipei male and Hualien female vegetarians than the matched omnivores. Lag time of LDL oxidation was negatively related to LDL arachidonic (r = -0.55, P = 0.0003) and eicosapentaenoic (r = -0.47, P = 0.003) acid contents. LDL-TBARS production was negatively related to LDL linoleic acid content (r = -0.36, P = 0.023), but positively related to LDL arachidonic (r = 0.56, P = 0.0002) and eicosapentaenoic (r = 0.45, P = 0.004) acids. No significant differences were found in dietary vitamins C and E intakes and plasma LDL alpha-tocopherol concentrations between vegetarians and omnivores. Our results suggest that vegetarian diets decrease the susceptibility of LDL to oxidation despite their higher dietary P/S ratio.