In a group of 26 preterm newborn infants (gestational age 24-37 weeks, birth weight 560-2800 g) we followed the possible relationship between the birth weight and the proportion of polyunsaturated fatty acids n=3 in the blood serum. The blood samples were taken usually within 24 hours after birth, exceptionally within 48 hours. The newborns as well as their mothers did not receive antibiotics, tocolytics or hormonal therapy. We found a positive correlation between the birth weight and the proportion (expressed in %) of polyenoic fatty acids(n-3) in the blood serum. This relation could be expressed by the regression line (r=0.636, p<0.001, % FAn-3 = -1.166 + 0.00217 x birth weight in g. The value of Kruskal-Wallis criterion H was 15.158. The importance of such a relationship is discussed.