Serum low-density lipoprotein (LDL) particle composition varies according to lifestyle and age. To analyze its long-term tracking, we studied LDL particle size consecutively in 100 children at the ages of 7, 9 and 11 years using a high-resolution 3% polyacrylamide gel tube, electrophoresis method, searching also for long-term determinants of the particle size. The mean LDL particle sizes at 7 and 9 years, and at 7 and 11 years correlated directly (r=0.72 and 0.39, respectively). The probability that children would remain in the same LDL particle size tertile between 7 and 11 years of age was 48% (p=0.008). Longitudinally, total, high-density lipoprotein (HDL) and LDL cholesterol concentrations and body mass index (BMI) associated directly with mean LDL particle size, and triglyceride concentration and triglyceride/HDL cholesterol ratio correlated inversely. A shift from pre-puberty to puberty was associated with an increase in LDL particle size. Sex, serum insulin concentration, or energy nutrient intakes did not associate with LDL particle size. In conclusion, although mean LDL particle size tracks in 7- to 11-year-old healthy children, changes in serum triglycerides, HDL, LDL, and total cholesterol concentration, BMI, and pubertal status all modify LDL particle size.