Backgrounds: Low-density lipoprotein (LDL) cholesterol is a proven risk factor for coronary artery disease (CAD). Recently, small dense LDL is reported to be more atherogenic and is now emerging as an important risk factor for CAD, especially in diabetic patients. However, datas regarding the relationship of LDL particle size and severity of coronary artery disease are limited in Korean population. Therefore, we investigated the relationship between mean LDL particle size and extent and severity of CAD. We also evaluated the LDL particle size in patients with acute coronary syndrome. Methods: Blood samples were collected from 261 consecutive male patients who underwent coronary angiography to evaluate chest pain from October 2003 to June 2004. LDL subfraction distribution was analyzed using a Quantimetrix LipoprintTM LDL System. The extent and severity of CAD were evaluated by reviewing the coronary angiography and measured by Gensini scores. The extent of CAD, Gensini scores, acute coronary syndrome and mean LDL particle size were investigated by multivariate analysis. Results: LDL particle size correlated significantly (P<0.05) with age, total cholesterol, triglyceride, HDL cholesterol and LDL cholesterol. Mean LDL particle size was smaller in patients with angiographically proven CAD than in controls (26.37짹1.10 vs 26.63짹0.79, p=0.035). LDL particle size in patients with acute coronary syndrome was even smaller compared with non-ACS patients (25.88짹1.74 vs 26.65짹0.54, p=0.002). Partial correlation coefficients controlling for age, total cholesterol, TG, HDL, LDL showed negative correlation between LDL particle size and Gensini scores (p=0.015). Conclusions: LDL particle size was smaller among patients who had more extensive coronary artery disease and moreover in acute coronary syndrome patients. Our data suggests that small dense LDL was independently associated with the incidence and extent of coronary artery disease and can be a risk factor for coronary artery disease and development of acute coronary syndrome in Korean male.