The fatty acid composition, including total trans fatty acid content, in human mature milk and serum from 16 Slovak women (8 vegetarians and 8 omni-vores) were determined using capillary gas-liquid chro­ matography. In human milk and serum from vegetari­ ans the mean percentage in total fatty acids of 18:2n-6 were 21.33% and 35.98% compared with 14.88% and 31.59% in omnivores. Long-chain polyunsaturated fatty acids such as 20:4n-6, 22:5n-3, and 22:6n3 were decreased in breast milk and serum from vegetarians compared with omnivores. Triacylglycerols make up 98% of the lipid content of milk. The fatty acid composition of the triacyl­ glycerols depends on the dietary fatty acid compo­ sition, and the carbohydrate -to-lipid ratio of diet [1]. Changes in the ratio of fat to carbohydrate in the diet lead to profound changes in the ratio of long-chain to medium-chain fatty acids in the milk, reflect­ ing an alteration in de novo synthesis of fatty acids in mammary gland [1-4]. Results of human milk lipid studies indicate that trans fatty acids from both diet and adipose tissue contribute to the total trans fatty acid content of milk lipid [3]. Trans fatty acids in­ hibit A 6 desaturation of 18:2n-6 and increase require­ ments for esential fatty acids (linoleic acid — 18:2n-6; a-linolenic acid — 18:n-3) especially of pregnant and lactating women [4]. The purpose of this study was to obtain infor­ mation about fatty acid composition, including long-chain polyunsaturated fatty acids, total trans fatty acid content (i.e., sum of all fatty acids which may have one or more trans double bonds) in human milk and serum from 16 healthy Slovak lactating women, 8 vegetarians practicing a vegetarian diet for 6.3±4.2 у and 8 omnivores, delivering at term, using capillary gas-liquid chromatography [3]. Mean maternal ages of the vegetarian and omnivorous groups were 29±5.7 (SD) у and 25.4±4 y, respectively. In 24-h collections of mature hindmilk from the breast after each feeding 50 different fatty acids were separated and quantified (wt % total fatty acids). Studied groups were com­ pared using Student's t-test. The fatty acid compositions of mature milk in veg­ etarian vs. omnivorous group were following: satu­ rated 37.14 vs. 39.78 %; cis-monounsaturated 33.15 vs. 37.77% (p<0.05); total trans-monounsaturated 4.61 vs. 3.86%; polyunsaturated 25.07 vs. 18.59%; palmitic acid (16:0) 16.78 vs. 20.64% (p<0.05); linoleic acid 21.33 vs. 14.88% (p<0.05); a-linolenic acid 1.34 vs. 1.27%; docosapentaenoic acid (22:5n-3) 0.11 vs. 0.16% (p<0.05); docosahexaenoic acid (22:6n-3) 0.15 vs. 0.22% (p<0.01). Linolenic/n6 ratio increased sig­ nificantly in vegetarian group. Vegetarian serum con­ tained less arachidonic acid (20:4n-6, 6.31 vs.7.4% , p<0.05), 22:5n-3 (0.39 vs. 0.53%, p<0.05), 22:6n-3 (1.23 vs. 1.47%, p<0.05) and slightly more total trans fatty acids compared with omnivorous group. The dietary intake of vegetarians contains a slightly lover proportion of energy from fat and sup­ plies fewer saturated fatty acids and more linoleic acid. Vegetarian diets provide only small amounts of long-chain polyunsaturated fatty acid such as arachidonic acid, eicosapentaenoic acid and docosahexaenoic acid containing in eggs and milk products. Changes in the fatty acid composition of the diet directly reflected milk and serum fatty acid compositions in studied groups. Mentioned results suggest that omnivorous in­ fants may get more long-chain polyunsaturated fatty acids (especially n-3), important for CNS development and retinal functions, than vegetarian infants do.