Familial hypercholesterolaemia (FH) is a monogenic disorder, with a strong family history, characterized by a deficiency in functional receptors for low density lipoproteins (LDL). The case of a patient with all the clinical traits of FH, including elevated cholesterol, xanthomas and early coronary and peripheral arterial lesions, but with a normal LDL receptor function, is described. In the patient the molecular weight and immunological properties of apolipoprotein (apo) B were normal; furthermore, autoantibodies to either LDL or to their receptor were also absent. The increased apo B/cholesterol ratio in LDL was compatible with the diagnosis of hyperapobetalipoproteinaemia. With the help of a turnover study using 131I homologous and 125I autologous LDL, it could be established that the patient had an almost three-fold increase in LDL-apo B biosynthesis, with, however, a fractional catabolic rate within normal limits. These findings pointed to the possibility of a genomic alteration in the region responsible for the control of apo B biosynthesis. However, extensive studies both at the cDNA level and in the 5' region of the apo B gene, failed to detect any significant alteration vs published nucleotide sequences. Although the exact mechanism for this unusual clinical presentation of an FH-like syndrome could not be uncovered, this case provides an extreme example of hypercholesterolaemia, with early and severe arterial disease, solely explained by an increased LDL biosynthesis.