We have studied the short-term intra-individual variation and long-term tracking until early adulthood of serum total cholesterol, low density lipoprotein (LDL) cholesterol, high density lipoprotein (HDL) cholesterol and triglyceride levels in 9-year-old Finnish boys. The study was based on two different cohorts; one to study short-term tracking and another to study long-term tracking. The short-term tracking cohort consisted of 57 healthy Finnish 9-year-old boys with serum lipid determinations 2 and 7 days apart in 1984. The other cohort consisted of 40 healthy Finnish 9-year-old boys and the serum lipid determinations were made in 3-year intervals from 1980 to 1989. Tracking was expressed as Spearman's rank order correlation coefficients between subsequent measurements. Tracking was weaker as the time between measurements became longer. For serum total cholesterol the correlation coefficients were 0.92, 0.77, 0.70, 0.73, and 0.59 for measurements made 2 days, 7 days, 3 years, 6 years, and 9 years apart, respectively. Other lipid variables behaved quantitatively in a similar manner. The dependence of tracking on time was not linear. The greatest relative decrease in correlations took place during the determination periods of 2 and 7 days, whereafter the decline was only moderate. In conclusion, short-term intra-individual variation of serum lipid levels in children, which is mainly due to biological variation, has a major influence on their long-term tracking. Our findings emphasize the need for several independent serum lipid determinations, performed at least one week apart, in the assessment of a child's true serum lipid level.