['This study sought to analyze whether the plasmatic level of leukocyte-derived microparticles (LMP) is associated with unstable plaques in patients with high-grade carotid stenosis.', 'Preventive carotid surgery in asymptomatic patients is currently debated given the improvement of medical therapy. Therefore, noninvasive biomarkers that can predict plaque instability are needed. The LMPs, originating from activated or apoptotic leukocytes, are the major microparticle (MP) subset in human carotid plaque extracts.', 'Forty-two patients with >70% carotid stenosis were enrolled. Using a new standardized high-sensitivity flow cytometry assay, LMPs were measured before thromboendarterectomy. The removed plaques were characterized as stable or unstable using histological analysis according to the American Heart Association criteria. The LMP levels were analyzed according to the plaque morphology.', 'The median LMP levels were significantly higher in patients with unstable plaque (n = 28; CD11bCD66b+ MP/μl 240 [25th to 75th percentile: 147 to 394], and CD15+ MP/μl 147 [60 to 335]) compared to patients with stable plaque (16 [0 to 234] and 55 [36 to 157]; p < 0.001 and p < 0.01, respectively). The increase in LMP levels was also significant when considering only the group of asymptomatic patients with unstable plaque (n = 10; CD11bCD66b+ MP/μl 199 [153 to 410] and CD15+ MP/μl 78 [56 to 258] compared with patients with stable plaque (n = 14; 20 [0 to 251] and 55 [34 to 102]; p < 0.05 and p < 0.05, respectively). After logistic regression, the neurologic symptoms (odds ratio: 48.7, 95% confidence interval: 3.0 to 788, p < 0.01) and the level of CD11bCD66b+ MPs (odds ratio: 24.4, 95% confidence interval: 2.4 to 245, p < 0.01) independently predicted plaque instability.'] Is plasmatic level of leukocyte-derived microparticles associated with unstable plaque in asymptomatic patients with high-grade carotid stenosis?