Previous studies have used relative (%) values of plasma 18:2n-6 (18:2 omega-6) and the ratio of 20:3n-9 to 20:4n-6 (triene/tetrene) to indicate the essential fatty acid (EFA: 18:2n-6 and 18:3n-3) status of preterm infants. The extent to which these indices reflect milligrams per liter quantities of n-6 and n-3 long-chain polyunsaturated derivatives of EFA (LCPs), required for cell membrane accretion, is unknown. Thus, 18:2n-6, the triene-tetrene ratio, and n-6 and n-3 LCPs were measured in plasma phospholipid of preterm infants' cord blood (n = 22) and of preterm infants fed formula (n = 12) or of preterm infants who received fat-free parenteral nutrition (n = 15) on postnatal day 3. Whether expressed as percent or milligrams per liter, 18:2n-6 was markedly higher and the triene-tetrene ratio lower in infants fed formula than in infants who had received no lipid. However, concentrations of n-6 and n-3 LCPs were similar in the two groups of infants and significantly higher than cord values. In the absence of an exogenous lipid source, it is suggested that tissue stores of LCPs are released in response to birth.