A total of 5000 consecutively samples newborn screening cards were anonymously selected for screening for the apolipoprotein B-3500 (apo B-3500) mutation, which causes familial defective apolipoprotein B-100 (FDB). The mutation was found in 5 of 5000 Danish children, of whom 2 were twins. This indicates a lower prevalence of this mutation in Danes than that reported in the UK, Germany, USA, Austria, Canada and especially Switzerland. Haplotype studies suggest that Caucasian subjects with the apo B-3500 mutation have a common founder. The apparently lower prevalence in Denmark than in Switzerland and Central Europe may indicate that the mutation was brought from these areas to Denmark after the initial settling of Denmark. In 101 unrelated Danish subjects with familial hypercholesterolemia, diagnosed on clinical and biochemical criteria including tendon xanthomata, 2 were heterozygous for the apo B-3500 mutation (2%).