Indoleamine 2,3-dioxygenase (IDO) is an important immunomodulator suppressing the activation of T lymphocytes, and its level in blood is increased in several autoimmune and inflammatory diseases. We have previously shown that this activity associates with several signs and risk factors of atherosclerosis in 24 to 39-year-old females. Now we repeat this analysis in an older population (n = 921, age range 46-76 years), i.e. in a population with more advanced atherosclerosis. IDO activity had a significant positive correlation in both sexes with carotid artery intima/media thickness (IMT), an early marker of atherosclerosis. In females, a significant negative correlation with HDL cholesterol and a positive correlation with triglycerides levels was observed. The association with IMT did not remain significant after adjustment with classical risk factors of atherosclerosis. It is thus concluded that IDO is a sensitive marker of atherosclerosis--or the inflammatory response associated with it--but does not have an independent role in the pathogenesis of this disease.