Background: Evidence for an interaction between Lp(a) and other conventional risk factors for cardiovascular disease, such as LDL, HDL cholesterol, and homocysteine have been found. However, the pathophysiological role and metabolic pathway of Lp(a) are not clearly defined. We investigated the relationship between serum Lp(a) level and LDL size in the patients with coronary artery disease(CAD). Furthermore, we investigated the correlations of small dense LDL(sd-LDL) and Lp(a) in the extent and the severity of CAD. Methods & Results: The 490 subjects (60.5짹11.5 years) who underwent coronary angiography to evaluate chest pain were investigated. Patients were classified into two groups: CAD group(n=256), who had significant stenosis in coronary angiogram. and Control group(n=234), who had normal coronary artery or minimal disease. The severity of CAD was measured by Gensini scores. The distribution of LDL subfraction was analyzed using a Quantimetrix Lipoprint LDL System and serum Lp(a) levels using the IgG-fraction of specific rabbit anti-Lp(a)-antisera. The Lp(a) level and mean LDL size were significantly correlated with the high Gensini score. LDL particle size in CAD group was smaller than that in control group (26.74짹0.64nm vs 26.43짹0.93nm, p<0.001). The serum Lp(a) level was not different between groups, but in the old aged patients(���55 years) the Lp(a) level was well correlated with the fraction of sd-LDL(R= 0.193, p< 0.001 ) and mean LDL size(R= 0.160, p= 0.003 ). Conclusion: Our data demonstrated that the LDL particle size was smaller in patients who had a CAD, and in old age subjects, the mean LDL size and the fraction of sd-LDL showed linear correlation with the serum Lp(a) level. The Lp(a) level was correlated with the severity and extent of CAD. Our data also suggests that there might be potential role of sd-LDL and Lp(a) in age dependent atherogenesis and the progression of atherosclerosis.