Atherosclerosis has a close relationship to inflammation, particularly T helper type 1 lymphocyte (Th1) response. Interleukin-10 (IL-10), is thought to suppress Th1 response. To target therapeutic strategy for atherosclerosis, we tested whether IL-10 gene transfer suppresses atherosclerosis in apolipoprotein E-knockout (apoE-KO) mice. Four-week-old apoE-KO mice were divided into two groups and either murine IL-10 cDNA plasmid or empty control vector was transferred to the femoral muscle with the use of Hemagglutinating virus of Japan (HVJ)-liposome. At 1 week after transfection, high cholesterol diet was started and continued for 8 weeks. After euthanasia, histological studies of atherosclerotic lesions and quantitative RT-PCR for Th1 cytokines (IL-12 and IFN-gamma) in spleens were performed. IL-10 cDNA gene transfer to the muscle increased plasma IL-10 levels and depressed expression of Th1 cytokines without changing plasma cholesterol levels. IL-10 gene transfer significantly reduced the atherosclerotic plaque area and the macrophage infiltrated area. IL-12 and IFN-gamma mRNA expressions in spleens and plasma IFN-gamma levels were decreased by IL-10 gene transfer. Therefore, IL-10 gene transfer changed the Th1 response and suppressed atherosclerotic lesion formation in apoE-KO mice. IL-10 could be a new target as a therapeutic tool for the treatment of atherosclerosis.