Clinical studies indicate that levels of apolipoprotein A-I (apo A-I), the major protein moiety of the high density lipoprotein (HDL) particle, may provide more information concerning the risk of future cardiovascular disease than do levels of HDL cholesterol (HDL-C). Therefore, the relationship of HDL-C to apo A-I levels was examined in a biracial sample of 2849 5- to 17-year-olds. The mean HDL-C to apo A-I ratio, a measure of HDL composition, was 0.42. However, marked interindividual variation was found: HDL-C/apo A-I levels varied from 0.27 (10th percentile) to 0.57 (90th percentile). Furthermore, only 26% of the variation in HDL-C levels was explained by concomitant variation in apo A-I. Increasing levels of triglyceride (and very low density lipoprotein cholesterol) were related to decreases in both the HDL-C/apo A-I ratio and the magnitude of the correlation between HDL-C and apo A-I. Low density lipoprotein cholesterol, race, and age were also related to the HDL-C/apo A-I ratio, but influenced HDL composition less strongly than did triglyceride levels. These observations may be explained by the bidirectional transfer of cholesteryl esters and triglycerides between HDL and triglyceride-rich lipoproteins. The current study documents the influence of triglyceride levels on HDL composition in a general population of children and adolescents, and emphasizes the interrelationships between the various lipid and lipoprotein fractions.