<b><i>Background:</i></b> Atherosclerosis is a chronic inflammatory disease of the artery wall where both innate and adaptive immunity play important roles. Modulation of the immune response against the stress protein antigen, heat shock protein (HSP) 60, by administration of mycobacterial HSP65 (mbHSP65) orally and/or nasally shows promising therapeutic results in young animals in the sense of less severe experimental atherosclerosis; however, the case of aged animals with already established atherosclerosis has so far never been investigated. <b><i>Objective:</i></b> To investigate if mbHSP65 immunization would further accelerate atherosclerotic progression in aged ApoE^-/- mice (18 months old) with already long-established atherosclerosis and if these mice could be orally tolerized against mbHSP65. <b><i>Methods:</i></b> Aged wild-type (WT) and ApoE^-/- mice (65 weeks) were immunized and/or orally treated with mbHSP65 and then either kept on normal chow or changed to high-cholesterol diet (HCD). Atherosclerosis was assessed by en face analysis and the number of CD4⁺CD25⁺FoxP3⁺ T regulatory cells (Tregs) was assessed by flow cytometry in lymph node and spleen cells. Total cholesterol and triglyceride levels were determined. Soluble mammalian HSP60 and anti-mouse HSP60 (mHSP60) and anti-mbHSP65 antibodies were detected by enzyme-linked immunosorbent assay. <b><i>Results:</i></b> As expected, aged WT mice had only minor lesions in the aorta, which did not change under HCD for 14 weeks. Aged ApoE^-/- mice already had large complicated plaques, which increased in size under HCD. mbHSP65 immunization led to a significant aggravation of atherosclerosis in both WT and ApoE^-/- mice irrespective of the nature of their diet. This increase was accompanied by increased titers of both anti-mHSP60 and anti-mbHSP65 antibodies in the circulation. The increased plaque formation could be significantly diminished with oral mbHSP65 tolerization. An increased number of Tregs and lower or unchanged levels of cholesterol and triglycerides were associated with the reduced size of aortal lesions. <b><i>Conclusion:</i></b> Oral tolerization against mbHSP65 could be used both to prevent and to treat chronic atherosclerosis in aged individuals.