The composition of fatty acids in serum lipids has been shown to reflect the quality of dietary fat. In order to examine the long-term stability (tracking) of the serum (and hence dietary) fatty acid composition serum cholesteryl ester fatty acids were analysed by gas chromatography in a randomly selected population sample of 1029 Finnish boys and girls aged 3, 6, 9, 12, 15 and 18 years, and again from the same individuals 3 years later. The tracking was evaluated by calculating linear correlation coefficients for the percentages of individual fatty acids between the 2 time points. For all fatty acids the correlations were highly significant, e.g. for linoleate and arachidonate they were 0.58 and 0.61, respectively. In 1980 the means of the percentages of some fatty acids showed certain differences between the various age and sex groups; in 1983 these differences had become smaller and had largely shifted towards three years older age groups, implying that the differences were due to dietary differences between the cohorts rather than to age and sex per se. The present results indicate that serum fatty acid composition has a fairly good biological reproducibility (tracking) and suggest that age and sex as such are only minor determinants of the serum fatty acid composition.