Background: High density lipoproteins are an heterogeneous population of particles. Two main subpopulations have been identified, one contains Apo A-I and Apo A-II and is denominated LpA-I:A-II and another one contains only Apo A-I and is denominated LpA-I. Aim: To measure the concentrations of these particles in patients with stable coronary artery disease. Patients and Methods: Serum lipids, A-I and B apolipoproteins, LpA-I, LpA-I:A-II and LpB particles were measured in 73 men aged 33 to 82 years with angiographically documented coronary artery disease (CAD) and 33 control subjects aged 39 to 76 years. LpA-I, LpA-I:A-II and LpB were measured by a noncompetitive enzyme linked immunoassay using previously characterized monoclonal antibodies against ApoA-I, ApoA-II and apoB. Results: Patients with CAD had significantly higher mean levels of LDL cholesterol than the control group (p= 0.038). The mean concentration of LpA-I particles in patients with CAD was significantly lower (p= 0.031) than in control subjects, while the concentration of LpA-I:A-II particles was significantly higher (p=0.016). The percentage of coronary stenosis correlated negatively with LpA-I and positively with LpA-I:A-II. The best relative risk (RR) indicator in these patients was LDL-cholesterol. The relative risk increases 2.5 fold when LpA-I falls below the cut-off level. Likewise, the relative risk increases 3-fold when LpA-I:A-II raises over the cut-off level. Conclusions: Our findings indicate that the quantification of LpA-I and LpA-I:A-II particles might allow a more accurate evaluation of the CAD risk than HDL cholesterol. LpA-I might represent the antiatherogenic fraction of HDL. (Rev Méd Chile 2000; 128: 9-16)