Cord serum prebeta-β-lipoprotein concentrations, was measured in a radial immunodiffusion assay in 303 randomly selected, full-term infants whose parental phenotypes were unknown. Six infants had elevated concentrations, that is above a cut-off limit of 197 mg/100 ml (97.5th percentile). Three of these infants suffered from classic type II hyperlipoproteinemia, and this diagnosis was confirmed by family studies at follow-up. The three other infants and their parents were shown to be normolipemic at follow-up. The elevated cord serum prebeta-α-lipoprotein in these three infants could be explained by highly elevated cord serum triglyceride and/or cholesterol. The 297 infants with normal cord serum prebeta-β-lipoprotein and their parents were all shown to be normolipemic at follow-up It is concluded that the measurement of prebeta-β-lipoprotein in cord serum allows the identification of children with familial type II hyperlipoproteinemia, if infants with transient neonatal hypertriglyceridemia and hypercholesterolemia are excluded.