A cross-sectional study of 4074 children, ages 5--17 years, in Bogalusa, Louisiana indicated significant changes of serum lipids and lipoproteins occurring during puberty. These changes could be related to both age and maturation and likely reflect the influence of sex hormones on serum lipoprotein metabolism. Considerable differences were observed in the race/sex groups, with boys showing a more marked decrease of serum total cholesterol, reflecting decreases of beta- and alpha- lipoproteins. While the ratio of mean beta-lipoprotein cholesterol/mean alpha-lipoprotein cholesterol for the white girls and black children remained relatively constant over the age span, the ratio beginning around age 11 years rose dramatically in white boys, reflecting an inordinate decrease of alpha-lipoprotein and an earlier beginning rise of beta-lipoprotein. The observations have important implications for earlier coronary artery disease in white men.