High density lipoprotein (HDL) and low density lipoprotein (LdL) cholesterol levels were measured in fasting blood samples from 950 healthy subjects and 188 aged patients by a new simple method. The HDL-cholesterol levels and HDL/LDL-cholesterol ratios are significantly higher in females than in males. In the healthy subjects, there are slight decreases in the levels of HDL-cholesterol and HDL/LDL-cholesterol ratio with aging in both sexes. The patients with myocardial infarction had significantly lower HDL-cholesterol levels and HDL/LDL-cholesterol ratios as compared to those of the group without infarction. On the contrary, no differences in total lipoprotein cholesterol levels were observed in the patients with cerebral infarction. The results, obtained in respect of electrocardiographic findings after the isoproterenol stress test, suggest that the HDL-cholesterol levels and HDL/LDL-cholesterol ratios may be related not only to the established myocardial infarction, but also to the presence of coronary atherosclerosis and stenosis.