The hyperlipidemias, with hypertension, diabetes mellitus and cigarette smoking, are amongst the major risk factors for the development of atheroma. The inter-relationships of hyperlipidemia and atheroma are complex but both appear to have a strong inherited component. Amongst the multiple genetic factors determining the common forms of hyperlipidemia, the apolipoprotein genes coding for the major peptides of the plasma lipoproteins (chylomicrons, VLDL, LDL and HDL) may be of particular relevance since the latter form a system of inter-converting particles for the delivery of lipid (triglyceride and cholesterol) to peripheral tissues (including the arterial wall). Recently several apolipoprotein genes have been isolated. Particularly interesting results have been obtained with the apolipoprotein AI and CIII genes. The DNA sequence of both genes and their immediate flanking region was determined. The two genes are physically linked and convergently transcribed. The cloning of the apolipoprotein genes made possible a detailed genetic study of patients with defects in lipid metabolism. An altered apo AI gene was shown to be inherited as a Mendelian trait linked to premature atherosclerosis in an affected family. Furthermore, the alteration of the apo AI gene seems to affect the expression of the apo CIII gene. Another DNA polymorphism that generates a new SstI site was shown to be present at low frequency (8%) in a random sample of the population. However, its frequency increased dramatically (42%) in a group of hypertriglyceridemic patients. It is thus not inconceivable that further studies of the genes involved in lipid metabolism will eventually help to replace the present phenotype based classification of lipid metabolism disorders by a genotype based system.(ABSTRACT TRUNCATED AT 250 WORDS)