Context: Low birth weight is associated with increased cardiovascular disease in adulthood, and differences in the molecular weight, composition, and quantity of lipoprotein subclasses are associated with coronary artery disease. Objectiv e: To determine if there are novel patterns of lipoprotein heterogeneity in low -birth-weight infants. Design, Setting, and Participants: Prospective study at a US medical center of a representative sample of infants (n = 163; 70 white an d 93 black) born at 28 or more weeks of gestational age between January 3, 2000, and September 27, 2000. This sample constituted 20%of all infants born during the study period at this site. Main Outcome Measures: Plasma levels and particle sizes of lipoprotein subclasses and plasma concentrations of lipids, lipoprotei ns (high-density lipoprotein [HDL] and low-density lipoprotein [LDL]), and apo lipoproteins. Results: An elevated lipoprotein peak of a particle with density b etween 1.062 and 1.072 g/mL was identified using physical-chemical methods. Thi s subclass of large HDL was enriched in apolipoprotein C-I (apo C-I). Based on the amount of the apo C-I-enriched HDL peak, 156 infants were assigned to 1 o f 4 groups: 0 (none detected), 17%; 1 (possibly pre sent), 41%; 2 (probably present), 22%; 3 (elevated), 19%. Infants in group 3, compared with those in the other 3 groups, had significantly (P<.001) lower m ean birth weight (2683.7 vs 3307.1 g)-and younger mean gestational age (36.2 vs 39.3 wk). After correction for age, infants in group 3 had signifi-cantly high er levels of total and large HDL cholesterol and of total and large LDL choleste rol and LDL particle number. However, infants in group 3 had lower levels of sma ll HDL, very low-density lipoproteins, and triglycerides than infants in the ot her 3 groups. This lipoprotein profile differed from that in infants born small for gestational age, who had significantly higher triglyceride (P<.001) and apo B (P=.04) levels, but lower levels of total and large HDL cholesterol (P < .001) and apo A-I (P<.001). Conclusions Because apo C-I-enriched HDL, and purified apo C-I alone, promotes apoptosis in vitro, increased amounts of this particle may have physiological significance and identify a novel group of low-birth-w eight infants apparently distinct from traditionally classified small-for-gest ationalage infants. Context: Low birth weight is associated with increased cardiovascular disease in adulthood, and differences in the molecular weight, composition, and quantity of lipoprotein subclasses are associated with coronary artery disease. Objectiv e: To determine if there are novel patterns of lipoprotein heterogeneity in low -birth-weight infants. Design, Setting, and Participants: Prospective study at a US medical center of a representative sample of infants (n = 163; 70 white an d 93 black) born at 28 or more weeks of gestational age between January 3 , 2000, and September 27, 2000. This sample constitutes 20% of all infants born during the study period at this site. Main Outcome Measures: Plasma levels and particle sizes of lipoprotein subclasses and plasma concentrations of lipids, lipoprotei ns (high-density Lipoprotein [HDL] and low-density lipoprotein [LDL]), and apo lipoproteins. Results: An elevated lipoprotein peak of a particle with density b etween 1.062 and 1.072 g / mL was identified Thi s subclass of large HDL was enriched in apolipoprotein CI (apo CI). Based on the amount of the apo CI-enriched HDL peak, 156 infants were assigned to 1 of 4 groups: 0 (none detected) , 17%; possibly pre sent, 41%; 2 probably present, 22%; 3 elevated, 19%. Infants in group 3, compared with those in the other 3 groups, had significantly (P < .001) lower m ean birth weight (2683.7 vs 3307.1 g) -and younger mean gestational age (36.2 vs 39.3 wk). After correction for age, infants in group 3 had signifi- cantly high er levels of total and large HDL cholesterol and of total and large LDL choleste rol and LDL Part number. However, infants in group 3 had lower lower levels of sma ll HDL, very low-density lipoproteins, and triglycerides than infants in the ot her 3 groups. This lipoprotein profile differed from that in infants born small for gestational age, who had significantly higher triglyceride (P <.001) and apo B (P = .04) levels, but lower levels oConclusions Because apo CI-enriched HDL, and purified apo CI alone, promotes apoptosis in vitro, increased amounts of this particle may have the sameness of life (P <.001) and apo AI and identify a novel group of low-birth-w eight infants apparently distinct from traditionally classified small-for-gest ationalage infants.