Background Inflammation is an important feature of atherosclerotic lesions, and the vulnerability of coronary lesions in acute myocardial infarction (AMI) at the time of onset may be related to blood levels of C-reactive protein (CRP) on admission, before CRP levels are affected by myocardial damage. Methods A total of 234 patients with AMI in whom plasma CRP was measured within 6 hours after onset were studied. They were divided into 2 groups: group 1 (n = 49) with elevated CRP (≥0.3 mg/dL) on admission within 6 hours after onset and group 2 (n = 185) with normal CRP (<0.3 mg/dL) within 6 hours after onset. All were treated by primary percutaneous transluminal coronary angioplasty with provisional stenting. Results There were no significant differences in baseline characteristics between the 2 groups. In-hospital adverse coronary events, including coronary reocclusion, reinfarction, target vessel revascularization, and death, were significantly more frequent in group 1 (22.4%) than in group 2 (4.3%, P <.005), and bailout stenting was performed significantly more frequently in group 1 (61.2%) than in group 2 (37.8%, P <.005). In contrast, there were no significant differences between the 2 groups in parameters that represent myocardial damage, including peak creatine kinase and left ventricular ejection fraction. Conclusion CRP levels within 6 hours after the onset of AMI reflect the vulnerability of culprit coronary lesions and predict adverse coronary events after primary PTCA/stenting. (Am Heart J 2000;140:324-8.)