The present multicentric study (three centers) deals with the values of plasma and red blood cell fatty acids obtained in a group of 18 preterm newborns after 2 days (D2), 15 days (D15) and 5 weeks (37th week postconception: 37th wk) of human milk feeding. Analytical methods were randomized between the three centers and quality control was evaluated by repeated analysis of reference samples. 20:4 n-6 varied from 10.71 +/- 1.58% to 9.51 +/- 1.65 and 10.10 +/- 1.42% in plasma phospholipids and from 16.59 +/- 3.30% to 14.68 +/- 3.14 and 18.24 +/- 4.09% in red blood cell phosphatidylethanolamine (RBC-PE) at D2, D15 and 37th wk, respectively, contrasting with the important rise of the precursor (18:2 n-6) in all the fractions studied. In RBC-PE, 22:6 n-3 significantly declined from 3.52 +/- 1. 03% at D2 to 2.56 +/- 0.83% at D15 (p < 0.02) and recovered its initial level at 37th wk (4.08 +/- 1.94%). The recovery of long chain polyunsaturated fatty acid homeostasis at that time was also confirmed by the decline of 16:1 n-7 in cholesterol esters (10.69 +/- 3.92 to 4.32 +/- 2.38%).