We investigated whether positive immunohistochemical staining of C-reactive protein (CRP) in initial culprit lesions is related to coronary plaque instability and whether it could affect the outcome of directional coronary atherectomy (DCA). The plasma level of CRP is a reliable marker of the risk of coronary events and restenosis after percutaneous coronary intervention. However, the influence of tissue CRP in atheromatous plaque on plaque vulnerability and restenosis remains unknown. Samples of DCA obtained from 12 patients with stable angina pectoris and 15 patients with unstable angina pectoris were immunohistochemically stained with a monoclonal antibody against CRP. We performed follow-up coronary angiography on 22 of 27 patients to evaluate the presence of restenosis after DCA. Immunoreactivity to CRP was localized to macrophages, smooth muscle cells, and necrotic areas. The ratio of CRP positive cells to total cells was significantly higher in DCA samples from patients with unstable (17.9 +/- 2.0%) than with stable angina (11.0 +/- 2.5%) (p <0.05). Follow-up coronary angiography showed that 12 of 22 patients developed restenosis after DCA. The ratio was also significantly higher in DCA specimens from patients with restenosis (19.3 +/- 2.8%) compared with those without restenosis (11.0 +/- 2.0%) (p <0.05). In addition, the ratio significantly correlated with late luminal loss (r = 0.428, p <0.05) and loss index (r = 0.636, p = 0.0011) after DCA. Immunoreactivity to CRP in coronary atheromatous plaque increases in culprit lesions of unstable angina, and it affects restenosis after DCA. These findings suggest that CRP in atheromatous plaque plays an important role in the pathogenesis of unstable angina and restenosis after coronary intervention.