Background High serum lipoprotein(a) (Lp(a)) levels are associated with coronary artery disease. Methods and Results The serum Lp(a) levels of 130 patients with acute myocardial infarction (AMI) who underwent direct percutaneous coronary intervention were investigated. On the basis of Lp(a) level at 1 month after the onset of AMI, the patients were classified into 2 groups (high-Lp(a) (≥30 mg/dl) and low-Lp(a) (<30 mg/dl)) for evaluation of the clinical coronary stenosis progression (CCSP) rate. CCSP is defined as either target lesion revascularization (TLR) or new lesion revascularization (NLR). The CCSP rate was significantly higher in the high-Lp(a) group than in the low-Lp(a) group (65.8% vs 29.3%, p<0.01). In patients who had coronary stents in the acute phase (n=79), the CCSP and NLR rates were significantly higher in the high-Lp(a) group than in the low-Lp(a) group (45.0% vs 20.3%, p<0.05; 35.0% vs 6.8%, p<0.01), but there was no significant difference in TLR rate between the 2 groups (10.0% vs 13.6%, p=0.858). Conclusions High serum Lp(a) level is a significant risk factor for CCSP, but does not influence restenosis after stenting. (Circ J 2006; 70: 156 - 162)