The composition of fatty acids in serum cholesteryl esters (CE) was analyzed gas chromatographically in 1348 boys and girls aged from 3 to 18 yr. A dietary survey was carried out simultaneously using the 48-h recall method. The dietary P/S ratio had highly significant correlations with CE fatty acids: positive with linoleate (0.567) and total omega 6 fatty acids and negative with saturated, monounsaturated, and omega 3 polyunsaturated fatty acids. The highest mean percentage of CE-linoleate was found in 15-yr-old girls (52.7, SD 4.68%) and lowest in 3-yr-old girls (48.1, SD 5.00%). Age, sex, and the degree of puberty had no independent effect on CE-linoleate after it had been adjusted for the effect of dietary P/S ratio by analysis of covariance. These results indicate that the fatty acid composition of serum CE depends on the quality of dietary fat and that CE-linoleate is a useful reflector of the dietary P/S ratio. The negative correlation between CE omega 3 fatty acids and dietary P/S ratio may be due to displacement of the omega 3 acids in serum CE by the much higher proportion of dietary linoleate.