In this study lipid and apolipoprotein patterns were investigated at birth and compared with those of adults. In cord sera, cholesterol, triglycerides, low-density lipoprotein (LDL) cholesterol and high-density lipoprotein (HDL) cholesterol were 38.2, 46.2, 50.5, and 31.9%, respectively, of adult values. Apolipoprotein AII, B and CIII were 48.6, 30.6 and 44.5% of adult values, while apo AI, apo CII and apo E showed values approaching those of adults (63.4, 73.3 and 89.7%, respectively). Also cholesterol/HDL cholesterol and LDL cholesterol/HDL cholesterol ratios were lower in newborns. In cord sera, lipids were correlated with various apolipoproteins in a surprisingly different way from adult sera. HDL cholesterol was not inversely correlated with triglycerides, and showed a highly positive correlation with apo E, apo CII and apo CIII, which did not correlate with HDL cholesterol in adults. These data supported the presence of significant differences in plasma concentrations and composition of lipoproteins at birth. Therefore HDL, apo CII, and apo E seem to play a different and more important metabolic role in neonatal lipid metabolism.