Conjugated linoleic acid (CLA) induces regression of preestablished atherosclerosis in the ApoE^–/– mouse. Understanding the mechanisms involved may help in identifying novel pathways associated with the regression of human disease. Animals were administered a 1% cholesterol diet for 12 wk, with 1% CLA supplementation from wk 8 to 12. ApoE^–/– mice fed only the 1% cholesterol diet for 12 wk were employed as controls. Transcriptomic analysis of mouse aorta showed that many of the components of the IL‐10 signaling pathway were modified during CLA‐induced regression. Real‐time PCR and Western blot analysis showed increased IL‐10 receptor expression, phosphorylation of STAT3, and downstream target gene expression in the aorta, alongside an increase in serum IL‐10 (79.8±22.4 vs. 41.9±5.5 pg/ml, n=10; P<0.01). CLA ‐supplementation also increased IL‐10 production in bone marrow‐derived macrophages (143.6±28.6 vs. 94±5.6 pg/ml, n=5; P<0.05). To explore the mechanisms for altered IL‐10 production, we examined the profile of monocyte/macrophage phenotype in the vessel wall, bone marrow, and spleen. CLA increased macrophage polarization toward an anti‐inflammatory M2 phenotype in vivo, increasing the population of Ly6C^lo monocytes (29 vs. 77±14, n=5, P < 0.05) in the aorta. CLA had similar effects on monocytes/macrophages differentiated from marrow‐derived progenitor cells and on splenocytes. The induction of IL‐10 on CLA supplementation in this model may reflect a systemic alteration toward an anti‐inflammatory phenotype, which, in turn promotes increased vascular infiltration by Ly6C^lo monocytes. These cells may contribute to CLA‐induced disease regression.—McCarthy, C., Duffy, M. M., Mooney, D., James, W. G., Griffin, M. D., Fitzgerald, D. J., Belton, O. IL‐10 mediates the immunoregulatory response in conjugated linoleic acid‐induced regression of atherosclerosis. FASEB J. 27, 499–510 (2013). www.fasebj.org