Background: It is well known that atherosclerosis is characterized by chronic inflammation of an injured intima and the pathological processes are initiated by accumulation of morphologically distinct, modified forms of low density lipoprotein (LDL). However, it is not well known whether the level of modified LDL has clinical significance underwent percutaneous coronary intervention (PCI). Method: Forty two patients( 62.4짹12.7 years, male:female=26:16) who underwent PCI were enrolled. Twenty healthy persons were evaluated as a control group. In order to minimize the interrupting factor of MACE, inclusion criteria included lesion length less than 25 mm, post-PCI MLD larger than 2.0 mm. Modified LDL was expressed semiquantitatively by agarose gel electrophoresis using charge modification frequency (CMF). Clinical and coronary angiographic data were analyzed. Result: Clinical diagnosis was stable angina in 4 patients, unstable angina in 16 patients, non-ST elevation myocardial infarction in 5 patients, ST elevation myocardial infarction in 17 patients. The patients with acute coronary syndrome had higher CMF than control group (4.59짹10.28 vs. -0.32짹5.42, p=0.013). There were no significant differences of CMF according to sex, presence of diabetes and hypertension. There was tendency of increased CMF according to type of hypercholesterolemia (IIa, IIb, IV, V). There was no significant relationship between CMF and lesion type by ACC/AHA classification. Minimal lesion diameter (MLD) was smaller in patients with higher CMF (CMF���10: CMF<10=0.46짹0.21 mm : 0.68짹0.52 mm, p=0.048). In patients whose LDL was less than 100 mg/dL, CMF with acute myocardial infarction is higher than CMF with unstable angina (5.32짹13.38 vs. -3.10짹7,72, p=0.033). Conclusion: The higher level of modified LDL is associated with the severity of clinical findings and stenosis in patients with acute coronary syndrome.