Familial defective apolipoprotein B-100 (FDB) is a dominantly inherited disorder caused by the substitution of glutamine for arginine at position 3500 in apo B-100. The presence of mutant apo B-100 in low-density lipoproteins (LDL) markedly reduces their affinity for the LDL receptor, leading to hypercholesterolaemia and increased proneness to coronary artery disease. In some FDB heterozygotes the clinical picture is indistinguishable from that in heterozygous familial hypercholesterolaemia (FH). In European and N. American populations the frequency of FDB is at least as high as that of FH. In most lipid clinics, 2-5% of patients given a clinical diagnosis of FH have FDB, not FH. Most FDB heterozygotes respond well to drugs that lower plasma LDL levels by inducing receptor activity. This may be due partly to increased receptor-mediated hepatic removal of mutant and normal precursors of LDL, using apo E as recognition element. Several important lessons can be learnt from the study of FDB.