To investigate early detection of young families with inherited dyslipidemia, we assessed changes in circulating apolipoprotein (apo) B and A-I levels and the apo B/A-I ratio during the 1st year of life and their relations to parental values. After measuring initial dried blood spot capillary blood levels in 919 babies when aged 4.25 +/- 0.98 d (mean +/- SD), we recalled at a mean age of 8.5 +/- 2.3 mo those with levels in one or more of the following categories: the top 5% of apo B values (group 1), the top 5% of apo B/A-I ratios (group 2), the bottom 5% of apo A-I values (group 3), and a group of 18 infants (group 4) who were not in any of the above three categories. We thus obtained serum lipid measurements in 51 infants and their parents. Two of the 13 group 1 infants had persistent hyperapo B at 8.5 mo; there was an inverse relationship between apo B and apo A-I levels in this group (p less than 0.001). Two of 11 babies in group 2 had lipid profiles consistent with familial hypercholesterolemia. However, none of the infants in group 3 had evidence at 8.5 mo of abnormal apo A-I or HDL levels, nor did their parents. For groups 1, 2, and 3, there was at 8.5 mo a regression of apo B and A-I toward mean levels for the "normal" 18 babies of group 4. In contrast, the high apo B/A-I ratios in group 2 remained high (p less than 0.01) and showed evidence of tracking.(ABSTRACT TRUNCATED AT 250 WORDS)