Background Elevation of C-reactive protein (CRP), among other markers of inflammation, is associated with an increased risk for cardiac events in patients with known coronary diseases and in apparently healthy individuals. Moreover, in patients with acute coronary syndromes, elevated serum levels of CRP are strongly predictive of the risk for death from cardiac causes. The purpose of this study was to investigate whether mechanical rupture of an atherosclerotic coronary plaque during elective stent implantation in patients with stable coronary artery disease (CAD) at low risk will cause a significant increase in serum levels of CRP. Methods and Results We measured serum CRP levels in 40 patients. Group 1 consisted of 12 consecutive patients with stable coronary disease who were at low risk, before and after elective coronary stent implantation. We compared the results in these patients to those of patients in 2 control groups: group 2 consisted of 12 consecutive patients with non-ST-segment elevation acute coronary syndrome (NSTSE ACS) who were undergoing coronary stent implantation, and group 3 included 16 consecutive patients with stable or unstable CAD who were undergoing diagnostic coronary angiography only without PCI. Peripheral blood samples for CRP level testing were withdrawn before percutaneous coronary intervention or angiography at the completion of the procedure, and 6, 20, and 48 hours thereafter. All patients with stable CAD (group 1) had a significant and uniform increase in serum CRP levels after elective stent implantation. The low mean baseline serum CRP levels increased 4.9 ± 4.1-fold 20 hours after coronary intervention (2.1 ± 1.2 before, 7.9 ± 3.4 after, P <.002). The baseline CRP level was much higher in the patients with unstable coronary syndromes (group 2). In this group, only a 2.1-fold increase in mean CRP level was observed after stent implantation (7.4 ± 5.5 before, 14.1 ± 9.6 after, P <.004). Also, the response in this group was less uniform when compared with that in the stable CAD group. By contrast, in patients undergoing diagnostic coronary angiography, the mean baseline CRP level was higher than in the patients in the group with stable CAD; however, the mean CRP after the procedure was not significantly elevated in this group (4.5 ± 3.6 before, 5.5 ± 3.7 20 hours after, P = not significant). Conclusions Mechanical disruption of an atherosclerotic coronary plaque during elective coronary stent implantation in patients with stable CAD who are at low risk causes a systemic inflammatory response expressed by marked elevation in CRP concentration. (Am Heart J 2003;145:248-53.)