High-density lipoproteins (HDL) are macromolecular complexes of lipids and proteins which float in the ultracentrifuge between densities of 1.063 and 1.21 g/l. A major division of HDL into two subclasses according to their specific density has been proposed: HDL2 (d= 1.063–1.125) and HDL3 (d= 1.125–1.21). The division is supported by the different proportions of lipids and apoproteins in the two subclasses and by evidence suggesting that the protective effect of HDL lies mostly in the HDL2 subclasses. Recent epidemiological, experimental and clinical data have supported a probable role or HDL as an anti-risk factor due to their negative correlation with the prevalence of ischaemic heart disease. Some of these data are presented and discussed in this paper.