Familial hypercholesterolemia (FH) and familial defective apolipoprotein B-100 (FDB) are autosomal codominant diseases characterized by elevated LDL cholesterol levels and premature coronary artery disease. Mutations of the LDL-receptor and apolipoprotein B genes, which affect the binding domains of their protein products, are the causal defects. Securing the diagnosis of these conditions by molecular assays is important because it mandates early intervention for coronary risk reduction. DNA screening for apolipoprotein B R3500Q gene mutation was performed in 913 unrelated Spanish individuals with a clinical diagnosis of FH using a modified polymerase chain reaction protocol and restriction enzyme genotyping. Thirteen FDB heterozygotes were identified (frequency of 1.4% in subjects with a clinical diagnosis of FH). The prevalence of hypercholesterolemic subjects with FDB in the general Spanish population was estimated to be as low as 2.8 x 10(-5) (95% CI, -3.1 x 10(-4) to 3.7 x 10(-4)). The ancestors of 11 out of 13 FDB carriers were from Galicia, a region of Celtic ancestry in Northwestern Spain. As the series included 100 unrelated subjects of Galician ancestry, FDB appears to be an important genetic cause of hypercholesterolemia in this region. All the R3500Q mutations were found on the same allele, assigned to haplotype XbaI-/MspI+/EcoRI-/3HVR48, suggesting that the mutant alleles are identical by descent in people from Spain, as observed in other Caucasian populations. In conclusion, the R3500Q mutation of the apolipoprotein B gene, a common cause of FH in central Europe, is infrequent in the general Spanish population, but it is common in Galicia.