Objective: Atherosclerosis is a chronic inflammatory disease regulated by T lymphocyte subsets. Th17 cells reportedly play important roles in the development of inflammatory and autoimmune diseases. In this study, we investigated the contributions of circulating Th17 cells and plasma Th17-associated cytokines to carotid artery plaques. Methods: Based on carotid artery ultrasonography, 280 atherosclerosis patients were categorized both by: (1) 4 levels for extent and severity of plaques (Level 1 = least severe; Level 4 = most severe) and (2) 5 groups for ultrasound features of carotid artery plaques (none, flat, soft, hard, ulcerated). Peripheral blood Th17 cell frequencies and plasma concentrations of Th17-associated cytokines (IL-17, IL-6, and TNF-alpha) were also determined. Results: For groups categorized by the extent and severity of carotid artery plaques, Th17 cell frequencies, common carotid artery intima-media thickness (CCA-IMT), and Crouse scores were significantly increased in higher level groups (Levels 3 and 4) than in lower level groups (Levels 1 and 2), and plasma concentrations of IL-17, IL-6, and TNF-alpha increased with increased levels of plaque severity. The same pattern was found for groups categorized by ultrasound features of carotid artery plaques. The results of Pearson correlation and multiple linear regression analyses showed that both CCA-IMT and Crouse scores for carotid artery plaques were significantly and positively correlated with Th17 cell frequencies and plasma Th17-associated cytokine concentrations. Conclusion: These results suggest that increased frequencies of circulating Th17 cells and Th17-associated cytokines are correlated to the severity and progression of carotid artery plaques. (C) 2011 Elsevier Ireland Ltd. All rights reserved.