Transcription factor activator protein-1 regulates genes involved in inflammation and repair. The aim of this study was to determine whether transcription factor activator protein-1 activity in carotid plaques is related to symptoms, lipid accumulation, or extracellular matrix composition. Methods: Twenty-eight atherosclerotic carotid plaques were removed by endarterectomy and divided into two groups based on the presence or absence of ipsilateral symptoms (b1 month ago). Activator protein-1 DNA binding activity was assessed, and subunit (c-Jun, JunD, JunB, c-Fos, FosB, Fra-1, Fra-2) protein levels analyzed by immunoblotting. Distribution of c-Jun in plaques was analyzed by immunohistochemistry. Results: Plaques associated with symptoms had increased activator protein-1 activity and increased expression of c-Jun and JunD, as compared to asymptomatic plaques. Fra-1 and Fra-2 were present in equal amounts in both groups, whereas JunB, FosB, and c-Fos were undetectable. Activator protein-1 activity correlated with cholesteryl ester and elastin in plaques and decreased with age. Activator protein-1 activity did not correlate with collagen, calcified tissue, or proteoglycan content. Conclusions: Activator protein-1 is increased in plaques associated with symptoms. The correlation between activator protein-1 and cholesteryl esters suggests that high activator protein-1 is a marker of plaque vulnerability. Activator protein-1 expression can also reflect the activation of repair processes.