Atherosclerotic lesions contain T lymphocytes, which orchestrate adaptive immunity and regulate many innate immune pathways. This study examined the influence of Th1 and Th2 helper cell subsets on atherogenesis in two ApoE^−/− mouse strains, C57BL/6 and BALB/c, which display opposite T-cell subset polarizations. ApoE^−/− BL/6 mice showed predominant Th1-like immune responses on polyclonal stimulation of splenic CD4⁺ T cells and had IgG2a antibodies to oxidized low-density lipoprotein (a disease-related antigen) whereas ApoE^−/− BALB/c mice displayed predominant Th2 responses by CD4⁺ T cells and an IgG1 isotype response toward oxidized low-density lipoprotein. ApoE^−/− BL/6 and BALB/c mice consumed a high-cholesterol diet for 10, 16, and 24 weeks with equivalent cholesterolemic responses. The Th1-slanted BL/6 mice developed significantly more atherosclerosis in the aortic root and abdominal aorta at all time points compared with BALB/c mice, supporting a proatherogenic role for Th1 response. Progression of atherosclerosis was associated with increased levels of interleukin (IL)-6 in mouse serum and CD4⁺ T-cell culture supernatants and increased levels of the acute-phase protein, serum amyloid A (SAA). Both IL-6 and SAA levels rose significantly in ApoE^−/− BL/6 mice compared with BALB/c mice. The circulating cytokine milieu (IL-6) and acute phase reactants such as SAA may reflect alterations in the Th1/Th2 balance. The results presented here illustrate how genetically determined modifiers of both immune and inflammatory responses can modulate atherogenesis independently of lipid levels.