Cholesterol and apolipoprotein (apo) B contents and their relationship within serum low density lipoprotein (LDL) were examined in 2018 children, ages 8 to 17 years, from a biracial community. The levels of LDL cholesterol and LDL apo B showed significant race-related differences (blacks greater than whites) in both boys and girls and gender-related differences (girls greater than boys) in white children. These LDL measures associated inversely and significantly with both age and Tanner stage, more so in boys than in girls. The black-white differences in LDL measures persisted after adjusting for the covariates (sexual maturation, age, adiposity, oral contraceptive use, cigarette smoking, and alcohol use). The distribution of LDL cholesterol for a given range of LDL apo B varied considerably, despite a strong correlation (r = 0.91) between these variables, indicating that measuring LDL cholesterol alone does not accurately reflect LDL concentration. The ratio of cholesterol to apo B in LDL ranged from 1.01 (5th percentile) to 1.42 (95th percentile) among the four race-gender groups, suggesting marked interindividual variation in composition. That this ratio was significantly elevated in black children indicates the occurrence of relatively larger, less dense, and cholesterol-enriched LDL particles in blacks. Significant independent associations were noted between LDL cholesterol/apo B ratios and the levels of serum total cholesterol (positive) and triglycerides (negative), suggesting the influence of pool size of different lipoproteins on LDL composition. These observations may help identify a subgroup of children with apo B-enriched LDL particles who are potentially at risk for coronary artery disease.