The major structural components of high density lipoproteins were determined in the sera of 638 male employees aged 40 years and older. It was demonstrated that the HDL apolipoprotein A-I/HDL cholesterol ratio as well as the HDL apolipoprotein A-II/HDL cholesterol ratio are similarly correlated to a cumulative score of established risk factors for atherosclerosis. Most important, however, is the finding that the correlation of these ratios to the risk factor rating of atherosclerosis is found in subgroups with normal or elevated HDL cholesterol values. Furthermore, it is shown that the relative content of apolipoproteins A-I and A-II in individual HDL is partly dependent on the plasma concentration of HDL cholesterol and triglycerides. It is concluded that HDL composition may have an additional predictive significance for the development of atherosclerosis.