We measured by radial immunodiffusion apolipoprotein B (ApoB) as a genetic marker for familial hypercholesterolaemia in heel prick blood spot samples on filter paper routinely collected from 5000 3- to 5-day-old neonates for current screening programmes. Dried blood spot ApoB levels were distributed continuously and were 9% higher in female neonates than in males (0.246 +/- 0.085 versus 0.225 +/- 0.079 g/L of whole blood, mean +/- SD, p less than 0.0001). Neonates of birth weight under 2.5 kg had lower levels than the population mean in both sexes but levels did not change with birth weight within this range; in those with the birth weights greater than 2.5 kg levels increased with increasing birth weight. Sex and birth weight could account for 5.7% of the variability of ApoB. Levels had largely stabilized by day 3 of life. Of parents of neonates whose ApoB levels were among the top 2%, 45 families were available for study when the infants were aged 12.3 +/- 3.3 months. In 6 of these families there was a persisting elevation of ApoB both in the infants (levels greater than 0.7 g/L of whole blood) and in one parent who also had elevated ApoB and a lipid profile of familial hypercholesterolaemia phenotype, results indicating an ascertainment rate of between 1 in 365 and 1 in 830 of the screened population. These studies have defined the variables affecting neonatal ApoB levels and establish that neonatal screening for familial hypercholesterolaemia is feasible. Primary prevention of vascular disease in young families with the gene should be possible with this approach.