Aim. Inflammation plays a crucial role in the progression of atherosclerotic plaques. The aim of the present study was to investigate phenotypic and functional characteristics of plaque-infiltrating T lymphocytes associated with a complicated phenotype of carotid atherosclerotic lesions. Methods. Atherosclerotic plaques were obtained from 17 patients undergoing carotid endarterectomy and cultured to isolate infiltrating T lymphocytes. Blood samples were obtained from patients and from 20 sex- and age-matched healthy subjects. The presence of lymphocytes (CD3+ cells) within atherosclerotic plaques was determined by immunohistochemistry. Phenotypic characteristics and intracellular cytokine expression of plaque-infiltrating and circulating T lymphocytes were determined by flow cytometry. Cytokine levels in supernatants from infiltrating T cell cultures were evaluated by enzyme-linked immunosorbent assay. Results. A higher number of CD3+ cells was detected. in complicated than in uncomplicated plaques. Complicated plaques had higher percentages of tumor necrosis factor (TNF)-alpha- and interferon (IFN)-gamma- positive cells than uncomplicated ones, especially in CD4+ subpopulation. In patients the percentages of TNF-alpha-positive cells were higher in infiltrating than in circulating lymphocyte samples. Intracellular TNF-alpha, IFN-gamma, interleukin (IL)-4 and IL-10 expression resulted higher in circulating lymphocyte samples from patients than in those from healthy subjects. Supernatants of infiltrating T cell cultures from complicated plaques showed higher levels of TNF-alpha and lower levels of IL-4 than those from uncomplicated plaques. Conclusion. Our data provide new information on the presence of increased percentages of pro-inflammatory T lymphocytes in complicated plaques with respect to uncomplicated ones and support the concept of the key role played by activated T cells in the progression of atherosclerotic lesions.