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)-cholesterol. However, it is not well known whether the level of modified LDL-cholesterol has clinical significance in patients who underwent percutaneous coronary intervention (PCI). Subjects and Methods: Eighty seven patients (63.0짹11.1 years, 58 male) who underwent PCI were enrolled. Patients with stable or unstable angina pectoris were classified as group I (n=44, 62.4짹9.3 years), and patients with acute myocardial infarction were classified as group II (n=43, 63.6짹12.7 years). Modified LDL-cholesterol was expressed semiquantitatively by agarose gel electrophoresis using charge modification frequency (CMF). Clinical and coronary angiographic data were analyzed. Results: Clinical diagnosis was stable angina in 13, unstable angina in 31, non-ST elevation myocardial infarction in 5, and ST elevation myocardial infarction in 38 patients. There were no significant differences of CMF between two groups (3.0짹7.9 vs. 2.1짹10.9, p=0.671). Diameter stenosis was larger in patients with higher CMF (CMF���10:CMF<10=84.0짹10.4 %:78.6짹13.7 %, p=0.047). Six-month MACE had no relationship with CMF in group I. However, in group II, six-month MACE developed more frequently in patients with higher CMF than 10 [2(28%) vs. 2(5%), p=0.031)]. Patients with acute myocardial infarction whose CMF was higher than 10, had in-stent restenosis on follow-up coronary angiography (p=0.003). Conclusion: The higher level of modified LDL cholesterol is associated with severe clinical and angiographic findings, and poor prognosis in patients with acute myocardial infarction.