Regulatory T cells (Tregs) have been shown to protect against atherosclerosisin experimental models but their association with cardiovascular disease inman remains to be elucidated. To determine if the level of Tregs in thecirculation reflects the severity of atherosclerosis and predict the developmentof acute cardiovascular events in man. The study included 350 subjectswith incident acute cardiovascular events (coronary heart disease or stroke)and 350 matched controls recruited from the Malm ¨o Diet and Cancer study.Mononuclear leukocytes stored at −140ºC at the baseline investigation in1991−94 were thawed and Tregs analyzed by flow cytometry. Release ofIL-10 from activated leukocytes was measured by Mesoscale. There was nodifference in Tregs (defined as CD4+CD25+FoxP3+ or CD4+CD25+ cells) orIL-10 release between cases and controls. There was also no associationbetween Tregs and severity of atherosclerosis as assessed by the carotidintima-media thickness at baseline. Our observations show that circulatingTregs, defined as CD4+CD25+FoxP3+ or CD4+CD25+ cells, do not reflectthe severity of atherosclerosis and do not predict acute cardiovascular events.However, it should be emphasized that these findings do not exclude a role forTregs in human atherosclerosis